If you don't mind I'm going to chime in here as I seem to be having an incredibly similar issue that began a few weeks ago.
Up until then I have always managed fast uploads via Rclone, in the 300Mbps region but now I seem to max out at around 25Mbps. This issue seems isolated to Rclone as uploading via the GDrive browser is still around 300Mbps.
Like RobertusIT I am also seeing the drawn out sawtooth pattern on my speed graphs.
Interestingly enough this is affecting two different Windows machines on completely different networks, which makes me inclined to think something has changed at the Google end and is affecting the version of Rclone I have installed on both machines (v1.45 I believe)
Not sure if it's relevant but I'm using Rclone Browser in both instances.
I guess it's not impossible, but rclone browser is basically the same as using the commandline (the RC) because the GUI just translates clicks into commands for you. It doesn't interface on it's own.
But if you want to test then I guess it's as simple as doing a test with the commandline (use -P to get an idea of the speed) and see if you notice any significant difference.