Not sure if this is universal but I have just started uploading my photos to google photos with rlclone. What I have noticed is, regardless of the number of transfers, my file - only CRW raw files at the moment - hang at 100% for 10-15 seconds, I assume while the baackend confirms the file acceptance.
Is there any way to stagger transfers or could there be? This way, in these kinds of circumstances, the first few tranfers would use bandwith and then hang around waiting for a "commit" then the next few tranfers start and so keep the pipe a little more filled.
What I find at the moment is that even with 32 transfers I get all at 100% for a while and then one or two clear and new transfers start and then more complete etc.