Rust painfully slow

I'm using CLion with the new RustRover plugin.

A few weeks ago, when I was using the old Rust plugin, the Rust support in the editor became very slow. If there was a red squiggly under a syntax error and I fixed it, it would take literally a minute for the squiggly to go away. If typed the name of a struct variable and a dot, it would take a some amount of time for the prompt with the struct members to come up: sometimes a minute, sometimes instantly. Syntax highlighting was similarly slow.

It wasn't this way up to a few weeks ago; everything was fast. 

I tried the new RustRover plugin today hoping that it would help, but it's no faster.

I'm using the latest version of everything. I'm on a fairly recent M1 Mac.

What's the fix?