Hello @linckez,
Personally I found --drive-chunk-size 256M was the optimal setting when uploading multiple larger files (50GB+).
I can now second that. Finally (after almost 3 years!) I got the incentive to try it, after an rclone transfer I run almost everyday which used to run at ~5MB/s (limited by its internet connection of 50Mbps) suddenly started to crawl at 0.25MB/s (seriously).
Tweaking the --user-agent parameter, which was what I used to do in these situations (and used to solve it as Google is apparently throttling traffic for certain user agents) did not solve it this time. So I remembered this, interrupted the running rclone with ^C, and reran the exact same command but with a --drive-chunk-size 256M tacked at the end.
With just that change, speed went from those lousy 0.25MB/s back to the 5MB/s range -- and I'm happy again 
PS: my files here are in the ~300MB-1.5GB range, so it seems --drive-chunk-size 256M works great even for files significantly smaller than the +50GB you reported.
Cheers,
-- Durval.