I don't have any mods on my client, only optifine.
But actually, I've found the source of these lags and to be honest, that is pretty weird:
For some unknown reasons, my client doesn't support a specific clock (the one that uses command blocks with /fill redstone block and /fill stone, the old one).
I don't understand how come I lag because of this clock whereas in solo, nothing bad happens, my client support it, but not when it's on a server so, I've just made the same clock but with more ticks so that it doesn't lag anymore.
But i don't really think that it comes from this, but maybe some commands that my client doesn't support with this speed, each tick.
But well, anyway, I think I've fixed the problem, so it's fine, normally even though I still get some lags but I think they come from my own connection or then because many people are using the Internet or I don't know what, but don't think it's going to keep lagging, at least, not as much as earlier.
Thanks for helping me though and if ever any constant lags happen again, I'll tell you, at least if I don't succeed in fixing them ![]()