System message for a flagged post is “one of the fastest things” I’ve experienced and probably that exists on this planet, maybe after cheetah or a missile launch.
Here’s a suggestion to improve it’s speed, if I may, since I retired and I have plenty of free time during covid.
I suggest you keep track of the people pairing them according to their flagging preferences. That way you can forecast a flagging post and you need less different users to actually spend time to read and flag a post. For example if 5 flags are needed, your system only needs 4 flags from the people who have matching preferences regarding flagging.
We all know how forums work, but I happen to be in forum business over 16 years now professionally.