OK, so it seems to me that regardless of whether you send the files one-by-one or in bulk, they are in practicality handled one-by-one.
I've conducted a test in which I uploaded 1 file, 2 files, 3 files, 4 files, 5 files and 10 files in parallel and recorded them time it took. I did this 9 times for each number of files and recorded the median time in seconds. All the files are in fact the same file (approximately 3 MBs in size):
Here are the results:
1 file: 9.6 secs total - 9.6 secs per file
2 files: 19.7 secs total - 9.8 secs per file
3 files: 26.5 secs total - 8.8 secs per file
4 files: 34.1 secs total - 8.5 secs per file
5 files: 44.6 secs total - 8.9 secs per file
10 files: 84.1 secs total - 8.41 secs per file
The bottom line is that regardless of how many upload requests you send Google Drive (assuming you don't get a 403), the amount of time it would take per file is the same. In my case, 8-9 seconds per file.
I guess the upload requests are queued on Google's end.