Improve ffmpeg x11grab screen capture performance - ffmpeg

Question

I have been doing screen-only (no sound) capture using ffmpeg with libx264 for the encoding quite successfully on an old machine built around a Core2 Quad Q6600 processor. I now need to include audio in this, but the fans on this ancient machine are too loud. So, I found a fanless motherboard (https://www.asrock.com/mb/Intel/J5005-ITX/index.asp) that has an Intel Pentium Silver J5005 processor and decided to use this instead. The CPU's benchmarks put it in a similar bracket to the Q6600, and the general performance seems to be significantly better, presumably at least in part because it's now using DDR4 memory that's faster access.
However, the machine fails horribly at the screen capture. It's missing frames all over the place; I actually end up with video that's missing almost half the frames, and plays back at about double speed. Also, any audio is just messed up so badly I can hardly think how to describe it, best I can come up with is that I get perhaps a quarter second of sound then a few seconds pause (the video meanwhile is actually still playing back, albeit with no sense of time).
Some things occur to me that might be the cause, or cure, of my troubles, some of which I might be able to fix, others not so much. What other things should I try? (I'd prefer to avoid simply throwing money at the issue with random ideas that are baseless!)
1) perhaps the CPU lacks some "extensions" to the instruction set (I recall years ago some CPUs gaining MMX extensions") so that the CPU is fast at mundane computing but sucks at video encoding.
2) perhaps the fact that the old machine had a dedicated graphics card, while this new one is sharing main memory with the graphics system means that reading the screen pixels is much slower.
3) perhaps the fact that this new machine has a single DDR4 memory stick in it means that I'm forcing all the memory reads and writes for the computations through the same memory as is holding the screen, and that's too much (implying that adding an additional memory stick might jus possibly help?)
4) perhaps there's some bios setting that would allow more efficient sharing of video memory?
5) my favorite, perhaps there's a better compression library that I could use to get decent quality screen capture with much less CPU usage.
I should also note that I have tried this with -threads 0, and the CPU usage hovers between 100% and 200%; around 100% when the screen is static, and rising as I move windows around and otherwise create more output.
6) the motherboard claims to have some kind of hardware video encoder built into it. I haven't paid this any attention to this point, as I assumed it was for the purpose of taking HDMI input and encoding it, but maybe there's a way to use this, if so, what libraries might I need to get ffmpeg to do this.
Edits:
This is an off the shelf ffmpeg. I'm certainly willing to try building it myself if I have some idea what I should do different.
The motherboard claims to have hardware encoders, but I'm struggling to find out what they are (seems like it's an Intel chip called "UHD Graphics 605" but nothing I can find suggests ffmpeg can work with that)
command line right now has been (without audio):
ffmpeg -video_size 1280x720 -f x11grab -i ${DISPLAY}+100,100 -vcodec libx264 -f alsa -i pulse -acodec ac3 -threads 0 ./video$(date +%F-%H-%M-%S).mp4
Log from a short recording session is:
ffmpeg version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 7 (Ubuntu 7.3.0-16ubuntu3)
configuration: --prefix=/usr --extra-version=0ubuntu0.18.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
[x11grab # 0x561a723e5ac0] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0+100,100':
Duration: N/A, start: 1578693116.465807, bitrate: N/A
Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1280x720, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
Unknown decoder 'libx264'
simon#studio:~$ ffmpeg -video_size 1280x720 -f x11grab -i ${DISPLAY}+100,100 -vcodec libx264 -threads 0 ./video$(date +%F-%H-%M-%S).mp4
ffmpeg version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 7 (Ubuntu 7.3.0-16ubuntu3)
configuration: --prefix=/usr --extra-version=0ubuntu0.18.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
[x11grab # 0x558225bc29a0] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0+100,100':
Duration: N/A, start: 1578693132.513351, bitrate: N/A
Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1280x720, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 # 0x558225bcd360] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2
[libx264 # 0x558225bcd360] profile High 4:4:4 Predictive, level 3.1, 4:4:4 8-bit
[libx264 # 0x558225bcd360] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x1:0x111 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=4 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to './video2020-01-10-14-52-12.mp4':
Metadata:
encoder : Lavf57.83.100
Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv444p, 1280x720, q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc
Metadata:
encoder : Lavc57.107.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Past duration 0.806847 too large 256kB time=00:00:00.43 bitrate=4835.3kbits/s dup=16 drop=0 speed=0.207x
frame= 371 fps= 29 q=-1.0 Lsize= 639kB time=00:00:12.27 bitrate= 426.6kbits/s dup=16 drop=14 speed=0.971x
video:634kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.813096%
[libx264 # 0x558225bcd360] frame I:2 Avg QP:18.16 size:221502
[libx264 # 0x558225bcd360] frame P:93 Avg QP:14.97 size: 2007
[libx264 # 0x558225bcd360] frame B:276 Avg QP:20.13 size: 69
[libx264 # 0x558225bcd360] consecutive B-frames: 0.8% 0.0% 0.0% 99.2%
[libx264 # 0x558225bcd360] mb I I16..4: 44.6% 0.0% 55.4%
[libx264 # 0x558225bcd360] mb P I16..4: 0.2% 0.0% 0.3% P16..4: 0.7% 0.1% 0.1% 0.0% 0.0% skip:98.5%
[libx264 # 0x558225bcd360] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 1.0% 0.0% 0.0% direct: 0.0% skip:99.0% L0:50.9% L1:49.0% BI: 0.1%
[libx264 # 0x558225bcd360] coded y,u,v intra: 41.3% 37.5% 37.4% inter: 0.1% 0.0% 0.0%
[libx264 # 0x558225bcd360] i16 v,h,dc,p: 58% 41% 1% 0%
[libx264 # 0x558225bcd360] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 33% 30% 14% 2% 4% 4% 5% 3% 5%
[libx264 # 0x558225bcd360] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 # 0x558225bcd360] ref P L0: 59.2% 8.8% 25.5% 6.5%
[libx264 # 0x558225bcd360] ref B L0: 59.4% 39.0% 1.6%
[libx264 # 0x558225bcd360] ref B L1: 96.5% 3.5%
[libx264 # 0x558225bcd360] kb/s:419.29
Exiting normally, received signal 2.