CCP did a mass test of a new tech that allowed some 3k people to simultaneously participate in a battle with almost no lag. They already get those numbers with Eve (the last major war saw a battle with some 14k characters involved), but they have to slow the game down to do it (tic rate in Eve is once per second, in the largest battles that can slow down to more than once a minute). The new tech they mass tested doesn't slow the game down at all, it was bonkers.

It is certainly possible to have better net code when there's only a dozen people on a server. It's absolutely shameful that Tarkov's is as bad as it is.