I didnt wanna make that argument with no proof but it feels like it does fuck with hitreg
I mean realistically speaking there are soo many reasons or causes as to why hitreg is fucking up, hell it can even be the hitreg system itself they've chosen for the server that could be crap on it's own. There are way to many things that can alter the hitreg and the game being almost 17 years old doesn't help with it's extreme ancient engine that thankfully will be fixed in S&Box. At the moment it's virtually impossible to fix every issue hurting hitreg it's just not possible on a modded server. However you can nullify some of the damage or pinpoint some the easiest causes to remove that fucks with hitreg.
SpecDM and I have previous experiences with SpecDM causing major issues on other server has indeed on this server hurt the hitreg. I've personally noticed hitreg being completely fine at the beginning of a match when no-one is dead dead or in spectator but once people start participating in SpecDM and relaying more information to and from the server it's start to get worse especially on a higher pop day.
since it's two worlds shooting guns, respawning, fighting and all of that jazz. The game is trying to register two "servers" at the same time and is trying to make it register shots as smooth as possible with it gimping up along the way since the game engine is outdated as fuck and the hitreg can only do so much with alot of downloaded custom content on the server. Hell with certain grenades (Boogie Bomb moment) not activating or activating on a SpecDM player.
I would say let's temporarily remove SpecDM to see if it's fucking with the hitreg that bad so it's killing two birds with one stone as people want it removed to bring back more social experiences which I agree, Garry's Mod is a social party game it lives and dies on socialization