weird issue with occasional stuck queries

Hey,

I'm having a weird issue where a few times a day, any query that hits a
specific index (specifically a `unique` column index) gets stuck for
anywhere between 1 and 15 minutes on a LWLock (mostly
MultiXactOffsetSLRU - not sure what that is, I couldn't find anything
about it except for a pgsql-hackers list thread that I didn't really
understand).
Checking netdata history, these stuck queries coincide with massive
disk read; we average ~2MiB/s disk read and it got to 40MiB/s earlier
today.

These queries used to get stuck for ~15 minutes at worst, but I turned
down the query timeout. I assume the numbers above would be worse if I
let the queries run for as long as they need, but I don't have any logs
from before that change and I don't really want to try that again as it
would impact production.

I asked on the IRC a few days ago and got the suggestion to increase
shared_buffers, but that doesn't seem to have helped at all. I also
tried deleting and recreating the index, but that seems to have changed
nothing as well.

Any suggestions are appreciated since I'm really not sure how to debug
this further. I'm also attaching a couple screenshots that might be
useful.

spiral