CPU usage spikes after ZynAddSubFx GUI is opened (and selected VSTs)

In the latest 1.2.0 RC3 build once I click on Show GUI to display the interface of ZynAddSubFx after I've added it as an instrument on my new project, CPU usage spikes high enough to temporarily freeze the desktop and the audio to be on a temporary loop in case the track is being actively played (note that it does not happen when I do not load the interface yet; however, the layout of the GUI once it's loaded doesn't matter, it still causes usage spikes). It appears both on Task Manager and LMMS' own CPU bar on top of the screen. During the time that this happens the close button of ZynAddSubFx also doesn't work.

This hasn't happened before on my laptop (Windows 7 64 bit AMD - HP Pavillion G6 laptop) and I have not installed any new audio or Windows updates on it when I installed the latest build. I did install this on top of RC2, but would that make a big difference? I'll try to uninstall and reinstall later if that's the case, but right now that's the issue I've had with the instrument.

POSTSCRIPT: I have tried other VSTs to see if the issue happens beyond ZynAddSubFx, and to a lesser extent it does, but not immediately: a few playthroughs with the VST makes it abruptly happen, and from there on it doesn't disappear until you close LMMS itself. This specific VST (http://www.vst4free.com/free_vst.php?plugin=VSCO2_Violin&id=2540) played back fine on RC2 even though it consumes slightly higher usage than other VSTs I tried, but on RC3 I can't sensibly use it.

Start by reproducing the issue through the Show GUI entry point with ZynAddSubFx and the reported VST on Windows, comparing RC3 with RC2. Check whether opening the GUI causes the CPU spike, audio loop, and unresponsive close button; done means the affected plugins no longer cause these symptoms.

Written by the indexing model from the issue text.