HTTP API stops responding while writing file - performance

Question

I have written an API http server in Go using Gorilla Mux. It works well. One of the endpoints is for uploading files and saving them to an NFS share mounted to the server pod. The client is a Swift 5 app using Alamofire.
For smaller files we just use copy to write them from the request body. For larger files, we use a buffered stream reader and writer to perform the write, as we had issues with time outs and drops when just using copy.
However, when the write is happening, the server stops responding to all new requests. How can I change or optimize this code so that the server continues to respond as expected? See code here:
func uploadFile(w http.ResponseWriter, r *http.Request) {
vars := mux.Vars(r)
repoBase := "./repo/gkp-directory/"
folderName := vars["uploadFolder"]
fileName := vars["uploadFile"]
fileSize := r.ContentLength
//
// Check if we have a package. They can be large and require special handling
//
if folderName == "pkgs" {
defer r.Body.Close()
//
// If the content is more than 10MB write to temp cache then move it
// in a seperate goroutine to the repo storage
//
if r.ContentLength > 10000000 {
buf := make([]byte, 10000000)
tempFile, err := os.Create(repoBase + folderName + "/" + fileName)
if err != nil {
log.Println("ERROR: Failed to create file.")
log.Println(err.Error())
return
}
defer tempFile.Close()
for {
n, err := r.Body.Read(buf)
if err != nil && err != io.EOF {
log.Println("ERROR: Error creating file on NFS")
log.Println(err.Error())
return
}
if n == 0 {
break
}
if _, err := tempFile.Write(buf[:n]); err != nil {
log.Println("ERROR: Error streaming to NFS")
log.Println(err.Error())
return
}
}
tempFile.Close()
r.Body.Close()
} else {
//
// If the package is smaller than 10MB we should be safe to write it directly to
// the NFS backend with no buffer
//
outputFile, err := os.Create(repoBase + folderName + "/" + fileName)
if err != nil {
log.Println("ERROR: Failed to create file.")
log.Println(err.Error())
return
}
defer outputFile.Close()
written, err := io.Copy(outputFile, r.Body)
if err != nil {
log.Println("ERROR: Failed to create file.")
log.Println(err.Error())
return
}
if written == fileSize {
outputFile.Close()
r.Body.Close()
}
}
} else {
//
// Otherwise file is not a package. This means it is just a small text file we
// should safetly be able to write this to NFS with no issues or buffer
//
outputFile, err := os.Create(repoBase + folderName + fileName)
if err != nil {
log.Println("ERROR: Failed to create file.")
log.Println(err.Error())
return
}
defer outputFile.Close()
written, err := io.Copy(outputFile, r.Body)
if err != nil {
log.Println("ERROR: Failed to create file.")
log.Println(err.Error())
return
}
if written == fileSize {
outputFile.Close()
r.Body.Close()
}
}
}