I ran into a weird issue where tooltips on some QPushButtons seems to cause quite extreme UI hangs / lock-ups when rendering the tooltip.
This in the context of heavy CPU load on the system (e.g. 24 render threads running on a 12 core system), regardless the UI lock-up is pretty extreme (e..g up to 30s)
Steps to replicate
1) Download and Install Chaotica 2.0.10 (http://www.chaoticafractals.com/dist/chaotica_x64_v2.0.10.msi)
2) Run Chaotica 2.0.10
3) A window called the 'Parameter Browser' will pop up, click on the top-left thumbnail button.
(This will start a render going in the main window)
4) Move your mouse cursor to another thumbnail, then leave it still to allow a tooltip to pop up
5) Result: Chaotica UI will lock up for 10-30s.
If this doesn't happen immediately you may need to repeat steps 3 to 5 until it does.
You can open the Parameter Browser window again with CTRL+B in the main window.
The evidence for tooltips being the problem is that the UI locks up exactly when a tooltip would render, and the issue has been worked around by removing the tooltips.