Curious performance problem w/ Nuendo 11 and Altiverb bug

The last week I’ve been battling a performance issue on a film sound project I’ve been working on for a two months. Decent size Nuendo project (130 tracks, good selection of plugins, 90min runtime). Has worked very well with audio performance meter steady at around 50%.

In the last few days I must have made one change that I cannot pin point that now makes the project fail to run well. If I open Nuendo and load the project the audio performance meter is pegged at 100% and playback stutters. Also the Nuendo process runs the CPU steady at 30%, like it’s stuck in a loop. Restarting Nuendo, etc. makes no difference. However, if I reboot the system and start Nuendo, it’s back to working with audio performance meter at 50%. But once I quit Nuendo, I’ll have to reboot again to be able to work.

This is Nuendo 11.0.41 on a Win10Pro system with relatively up to date software all around. And hardware performance is not a limitation.

All seems to point to the session loading something that doesn’t completely clear/die after exiting Nuendo, but gets cleared with a reboot. I’ve checked task manager and there are no obvious left overs.

This coincided with two changes. One, I went to backup copy of the project from the previous night because of a change that got lost and I didn’t want to redo. And I added two effects channels, one with Altiverb and one with built-in multi-tap delay.

Any ideas what might be causing it? For now I have a work around, albeit an annoying one.

PS: The change got lost because of an Altiverb bug. I had added it to a 5.1 FX channel. Doesn’t quite work there. However, if you try to remove Altiverb from a 5.1 FX channel, or delete that channel, Nuendo crashes hard. That’s why I had to revert to an earlier project, as there was no way of fixing the project that Altiverb on it without crashing Nuendo while fixing it.