Regressed UI performance in VirtualBox Ubuntu guest in version > 3.4.1

I'm using QtCreator inside a Linux Mint 17.1 VirtualBox host (which is based on Ubuntu 14.04), on a Windows 10 guest, and I've been having trouble with the UI in all versions after 3.4.1. I've tested 3.4.1, 3.4.2, and 3.5.0.

My problems are with irratic and slow scrolling, seems like it misses a notch on the scroll wheel now and then. The framerate also seems lower in newer versions, which makes the UI less responsive.

I've tried setting the environment variable QT_GRAPHICSSYSTEM to native, opengl and raster, and it seems like opengl improves the problem somewhat. But 3.4.1 still performs a lot better than newer versions.