Latency when adding things to the GM Layer?

I'm using a custom map (not a lot of colors, pretty basic black and white) and everything is fine until I add enemies to the GM Layer to hide them. Once I do that, the map becomes very slow to move around (and move roll20 tokens on). If I leave them in the Objects layer it's fine.  Anyone else having this issue?  I should add that I made this map by pasting in an image of a map I wanted, right clicking and going to Advanced and selecting "Set Dimensions". It's 4500 px x 4500 px. I've made the same map via dragging the same map and then manually expanding it but on a map that's only 3500 x 3500. Is that my issue? 

That is an unusually large map (roughly 64 x 64 at 70 pixels per unit.) It could be really problematic if it is also saved in a lossless format like png. You can ensure some functionality by making sure to save as a jpg, at no more resolution than necessary and as much compression as you feel comfortable with. Also, you can slice a graphic into several pieces and fit them together in Roll20. I'm not sure how this would interact with tokens on the GM layer, though.

Thanks for this, Keith, I DID save it as png without thinking about it! I'll re-work it as a much smaller image and see how it goes. 

That worked, thank you!