Hi Nis, I haven't seen that problem over here. What is your hardware?

One of the big differences after the rewrite is that MoI v4 now supports Retina screen resolutions, that means that on a retina screen MoI draws the viewport at 4 times the resolution of v3. If you're on a somewhat older machine maybe that's enough extra work to bog things down a bit.

Do you see any difference if you right click the v4 app and choose "Get Info" and then check the "Open in Low Resolution" checkbox?

Thanks, - Michael