@ScytheX I'll give an example to better explain what I'm talking about.
Let's assume you have a parallel process that watches for a button press and checks several things in addition to that. And let also assume that you don't place waits in it at first.
The result is that the parallel process is checked sixty times per second and especially due to the included checks that requires a lot of processing power - more than the computer has in excess. As a result the entire game is slowed down and reacts clumsily to tries to get the button press in - that is lag (the real lag, slowdown due to missing processing power.
Now you've heard somewhere that waits reduce lag and add a wait(60) to that parallel process, because (also assumed) you don't understand what exactly that means.
As a result the button press is only checked once every second (sixty frames), and because the check is only for one sixtieth of a second - which means the player misses the button sequence very often.
At first glance the effect is the same (the computer doesn't react to every button press on time), but in the second case it is not because of lag, but because the computer has been instructed to only check the button once per sixty frames.
The solution in this case would be to reduce the number of wait frames, because what the wait command really does is to tell the computer to skip x frames before processing that event again, and since the event is not processed every frame that means less demand on the processor or less lag.
A good description of what really happens and for example if you have tried that on new maps without other events and so forth can give hints to see if your slowdown is really caused by lag or if it is one of those pseudo-lag forms where something has the same slowdown effect but without really slowing down the computer.
Another test that might be helpfull is to make a copy of the project with the problems and in that copy only, delete all common events, redirect the starting position to a new map and only place those events on the map that you need to test the effect.
And perhaps don't max out the levels in one step, but add some levels, test, add more levels and so on. and with each test check if there was an effect before continuing.
If the effect slowly increases then there is a good chance that it is real lag - if it suddenly jumps in with only a minor number increase near top it might be something else.