Wow, I'm just using the trial version but I'm having the exact same problem on a GTX 1070.  Runs absolutely terribly while the window is active.  Very strange.

Edit: I was getting around 3fps in the viewport before I changed my G-SYNC settings

I discovered something interesting, where the addition of one character slowed down the FPS to a crawl even in Quick Mode. I had used that character before without issues, but the one thing that I changed was the hair and that turned out to cause the problem. The hair is not very high-poly (about 13k), but it looks fuzzy so that may be the problem: lots of draws.

It also doesn't matter if I change the shading; even in wireframe it slows down playback.

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I noticed that when I moved my iClone 7 window to my other monitor the issue went away.  Turns out it was caused by my G-SYNC settings (as someone mentioned in a previous post).  These settings fixed the problem.  

Edit: Well, it fixed it for a bit but now I'm having problems again.  The frame rate is fine if I move the window to another monitor that doesn't have G-SYNC turned on.

HAHAHAHA Me searching for a Solution finds this!

I'll keep posting this but if your scene has more than a few dozen props that's mostly the issue.  If you consolidate them all (merge props) you will find a huge increase in speed).

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Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley