It can be client-side and still affect everyone equally. E.g., imagine a line of code (client side): "slow game down, if 10 players within 1km distance". Most of the 11 players would be experiencing that slowdown when they group up. And if a squad moves together, they would all be affected equally.
Of course, the reality is probably not a line of intentional bad code, but the idea is that client-side doesn't mean that everyone can't be experiencing the same thing at the same time.
IF this problem IS actually client side and PC-dependent then it is a very serious problem giving a huge disadvantage to anyone with weaker PC's.