well that last one is fine.... when it's actually shooting as flying ones/spires, but if there are none on the field that it gets VERY laggy even with just a couple hundred monsters, let alone the thousands and tens of thousands that some people have... still, avoid at all costs when you have 100+ monsters on the field...
It depends on cpu mostly, even random and killing off screen I get some lag with 1k monsters/wave when hitpoints are more than 10^30.
What patch version are you playing as well? If it's earlier than 1.0.18, tower gems with more than 100 fire rate (i.e. anything useful) are stupid laggy even with one monster to shoot at.