It really depends on your configuration of Plex mostly.
Plex does some basic scanning that will be decently fast via rclone - but it also some some more advanced scans that involve gathering metadata. This will be much slower at it will at least have to actually read parts of each file to find non-basic data embedded in the media file. Some deep-scans even analyze the entire file - and what that is done from cloud it means downloading the entire file. That can obviously take a very long time on a large archive...
TLDR: You will want to very carefully look at what scan settings you use. Not every type of operation that is viable on a HDD will work so well on a cloud-drive.
I have a decent idea of what types of scans will cause problems - so I can probably make some good guesses if you post a pic of your scan config from Plex. That said, I don't actively use Plex myself yet so I am not an expert in this. To get conclusive answers on Plex settings rather than my educated guesses you will need someone else to chime in. Thankfully there's no shortage of Plex users here.
Again, I'm not an expert on this - but I don't see any problems in that config.
In fact it should be ok to enable autoscan if you also use partial scan. This should only use basic metadata so this should be tolerably fast, and it will allow new shows to be added automatically which is probably quite useful.
But there is another page that deals with maintenance tasks and stuff like that. That has a lot of settings related to deep-scans of metadata ect. See if you can find that page, as I think that's where the worst offenders are located.
You may also find this thread useful as you will find a ton of fellow Plex users there:
Just be aware that this isn't a general Plex help thread, so try not to derail it from the topic - but it has a lot of relevant info + it makes it easy to connect with other users who have direct Plex experience. All the info I give you here is just my best guess. I have a fairly deep understanding how how rclone works but I can mostly just make educated guesses on how the Plex options interact with rclone.