I was having 429 Rate Limit Exceeded errors in my log. It was taking 20-30 seconds to access files. I found this thread on the RClone with other users having the same problem. The fix is to add the user-agent option to your mount script.
I added this --user-agent "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.77 Safari/537.36" which is the Chrome user-agent. This has fixed the problem for me. There are reports that you can choose any user agent you want and it will work.