being IO busy means your disk can't sustain what's being asked to do. You've posted a sequential write test, which you can see in the screenshot, your disk IO is read, not write and the box on the right so you are IO bound.
Sorry before I had to go, I did not paste the read speed from the bench script:
ioping: seek rate
min/avg/max/mdev = 194.8 us / 3.43 ms / 73.8 ms / 5.39 ms
ioping: sequential read speed
generated 1.91 k requests in 5.00 s, 478 MiB, 382 iops, 95.6 MiB/s
As you can see there is not any disk issue.
This is why I came to the forum, to find out any help as if the disk (read and write) is okay and download/upload speed is also ok... it should be the rclone mount what is failing.
This won't help you, but OVH really gets you on the network speed. The servers with gigabit are much more expensive, but you'll have to pay if you want to stream at the highest bit rates.
I've been paying $95 a month for a dedicated server with OVH in Canada since 2017. I do it mostly for the convenience of fast speeds, unlimited up- and downloads, no hardware maintenance on my end, etc.
No issues whatsoever with Plex streaming to both Europe and California.