This definitely happens when I making spawn trap. Thus far I have not tried it to replaying it from the start. The only thing I have from scratch is the cave part which also had a ton of slimes which you stand no chance against since it spawns a ton of them all at once, but luckily after building a structure in the middle part they get stuck underneath trying to reach it forever.
I would say to fix the balance there are two ways of what I am thinking:
Either spawn a bunch of them but have them squishier to kill or have less of them at the health they currently are, but have them spawn 2-5 at a time not all of them at once, so it's like a constant wave for a good 20 or more seconds. Depending on the defences the waves start hard and get easier as the player advances and has better ways of dealing with them.