LOL Hoel... it's not that bad on my computer.
My maps tend to slow down a bit when there's 100+ entities. It gets worse and worse from there. The Concetron map has thousands of entities for example
Most of my maps are 1000x800.
There's a few way to increase performance. I never have effects on while actively building and you can decrease some effects. See my second set of tutorials how to do that. It will be a lot faster. Unfortunately the eye candy suffers of course.
CAD programs are known to require insane amounts of resources