When I use the mod on 1.12 servers my game freezes for about half a second every time I load chunks, and I get ~150 fps less than normal when not loading chunks with occasional stutters.
I am using the latest version of fabric and multiconnect, with no other mods installed.
This is not a pc issue as I have an i7 8700k and a 1070ti, and 4 gb of memory allocated to minecraft.

Are there any significant performance problems as of the latest version? Performance has been improving over time for a while

Unfortunately, yes. For some reason, this affects the FPS directly.

If you're having performance issues, could you post your PC specs? I'm specifically interested in how many cores your processor has, as multiconnect uses that information to tune how many threads to use for chunk translation.

Intel(R) Core(TM) i5-8300H (up to 3,99GHz) (4 cores, 8 threads)
Nvidia Geforce GTX 1050
16GB RAM
This is strange. I retested Multiconnect today and it didn't cause any problems (except for spam in logs with various errors). Perhaps I changed some settings of my client that negatively affected performance.

This issue is unlikely to persist after the rewrite. Please reply if it does.

Are you sure you are using the latest version of multiconnect, 1.3.30? Issues like this were fixed a while ago. Chunk translation is an intensive task and can cause fps drops, but it shouldn't be freezing the game for half a second.

Yes, I downloaded it today.

Which server are you playing on where you are having this issue? Can you reproduce it on other servers?

2b2tpvp.net, I tried a few other servers and only 1 other had issues, so it might be the ammount of players in render distance or a plugin that is causing it

Is there a server I am likely to be able to login to to test this, that maybe isn't 2b2t related?

I don't know any 1.12 servers that aren't 2b related, but you could host your own locally while you test

I have never had this issue myself since the performance issues were fixed several versions ago. I'll test on 2b2tpvp.net in my own time. Is there any particular area on the server this happens?

Everywhere, but mostly near the spawn area where most of the players are

^ Tried to reproduce this performance problem and confirmed on specific 1.13.2 protocol and older versions. (Tested on play.cubecraft.net server with version manually set)