for some time i havent been able to play on the servers, because of skyrocketing ping, i couldnt tell why that was, it was fine in 0.6 and then 0.8 i got lvl 100 on official without any of these problems, stopped playing for some time and i think in 0.9 it was unplayable, definitely before the 1.0
now i noticed opening pretty much any interface, inventory, backpack, chests, crafting table, even escape menu lags me to the point i get timed out, dont even have to do it excessively, opening inventory and attempting to craft something is met with lag, opening a couple of chests is pretty much a timeout, no problems in single player
my best bet is either adding or making changes to What Are They Up To (Watut) combined with poor internet connection might cause that, maybe something else, but servers without the modpack dont lag at all when spamming inventory like in the videos, one of the videos is in tristram official server, other is mmcn
https://www.youtube.com/watch?v=KLpiHT0uzhY&ab_channel=disunio
https://www.youtube.com/watch?v=r5WUQHRiZcQ&ab_channel=disunio