iMac 5k Fullscreen Terminal/Opacity Slow & Sluggish

Hi,

i have a weird issue with my new iMac 5k that i have narrowed down specifically to the size of windows with opacity.

The issue at first was not very clear, but at certain points when using my iMac 5k, the whole computer would get sluggish. Memory usage was mimimal (its got 32gb ram) and so was CPU usage (top i7), but things like new browser tabs, switching between finder and another application just by clicking it in the background, mission control etc. were all very slow in responding. The issue would not go away until i restarted my computer.

After a day of trying to investigate what was causing the issue, i managed to narrow it down to Terminal. I have a home-brew theme with 90% opacity set, and i worked out that as soon as i resize the terminal window to fill at least 70% of the screen, the sluggish issue would start. The problem is that even when i quit terminal it would continue. The only fix would be a restart.

I use terminal all the time, so the clear fix for the issue is to change to a theme without opacity (which i have done), but there must be some problem going on in the background if i can't have a basic window filling more than 70% of the screen with any sort of opacity. I realise the 5k has to power a lot more pixels, but wanted to check that its not some other issue.

Thanks

Ali