Slowdown when using Mutect2 container inside Nextflow - bioinformatics

Question

I'm trying to run MuTect2 on a sample, which on my machine using java takes about 27 minutes to run.
If I use virtually the same code, but inside Nextflow and using the GATK3:3.6 docker container to run Mutect, it takes 7 minutes longer, for seemingly no apparent reason.
Running on Ubuntu 18.04, the tumor and normal samples are from an Oncomine panel. Tumor is 4.1G, normal is 1.1G. I thought the time might be spent copying in data to the container, but 7-8 minutes seems far too long for that. Could it be from copying in reference files too?
bai_ch is the channel that brings in the tumor and normal index files
process MuTect2 {
label 'mutect'
stageInMode 'copy'
publishDir './output', mode : 'copy', overwrite : true
input:
file tumor_bam_mu from tumor_mu
file normal_bam_mu from normal_mu
file "*" from bai_ch
file mutect2_ref
file ref_index from ref_fasta_i_m
file ref_dict from Channel.fromPath(params.ref_fast_dict)
file regions_file from Channel.fromPath(params.regions)
file cosmic_vcf from Channel.fromPath(params.cosmic_vcf)
file dbsnp_vcf from Channel.fromPath(params.dbsnp_vcf)
file normal_vcf from Channel.fromPath(params.normal_vcf)
output:
file '*' into mutect_ch
script:
"""
ls
echo MuTect2 task path: \$PWD
java -jar /usr/GenomeAnalysisTK.jar \
--analysis_type MuTect2 \
--reference_sequence hg19.fa \
-L designed.bed \
--normal_panel normal_panel.vcf \
--cosmic Cosmic.vcf \
--dbsnp dbsnp.vcf \
--input_file:tumor $tumor_bam_mu \
-o mutect2.somatic.unfiltered.vcf \
--input_file:normal $normal_bam_mu \
--max_alt_allele_in_normal_fraction 0.1 \
--minPruning 10 \
--kmerSize 60
"""
}
My only thought is to create my own docker that has the reference files handy, which will probably save time for copying them in? I'd expect the nextflow+container version to run only slightly slower than the CLI version.

Answer 1

Check the task Bash wrapper in the task work dir to asses the performance issue.