Manekk has posted in our Forums too, particularly we were trying to figure out another known problem: stutter or input lag

Regarding this topic's issue though, I've read your bigger reply and the first thing that I wanted to know was if that sluggishness that you notice in big close-quarters cluster-fucks can be experienced in Singlepayer as well.

Putting the "there is a problem with your connection" Multiplayer error aside for a moment, it would be interesting to know whether the machine has some fault into this sluggishness (by playing Singleplayer you remove the network aspect entirely)...

... or if it's otherwise all attributable to anything taking part to the network pipeline (so starting from your own Computer's network card, down to the medium that connects you to your Router, finally to the Internet Service Provider).

Manekk speaks about interpolation in the Discourse page you linked (by the way the WTFast page is also interesting, I know that service since battlelog but I don't recall seeing that page before) so, if this close-quarters bad user experience is not reproducible at all in Singleplayer, perhaps you could try to customize your Usersettings.con file to match your average ping.

Digression about Usersettings.con and my own (bad) bf2 hit registration experience

It's a file in the game directory, that models lag compensation, interpolation and extrapolation, but the fact of the matter is that it has default values that made sense with the network capabilities of 15 years ago, when the game was released. Personally I've always had average to bad internet connections, but even when upgrading them I would still experience my bullets not connecting to an enemy (especially with moving targets), enemies killing me instantly instead or worse, killing me after I took cover by ducking or by running behind a corner... I'm not the best player I know, but I'm not the worst either, so these issues about bad network hit registration negatively influence my gaming experience, especially when I feel that "damn my aim was on point but I still missed" or "uff, I reacted in time to the enemy shooting me, took cover, but I still got killed as if the bullet curves like in the Wanted movie and reaches me behind the corner too".