I don't know about Nuendo, but check to see if there is some sort of performance indicator that monitors your CPU and Drive usage. (there's one in Cubase) GENERALLY, if you're maxxing out your CPU, you need to conserve on effects, EQ, etc. If you're maxxing out on your drive, you have too many tracks for your computer to handle. Given most of today's computers, having only sixteen tracks choking your computer would be surprising. I have a Celeron 466, and I have run 32 tracks of audio with 18 channels with EQ and compression simultaneously.
Your strategy of bouncing tracks together could be a very workable solution to either problem, but there are a few other things to consider first.
If you need to conserve on CPU power, you can limit the number of effects and EQ you're using by using the aux sends for effects rather than inserts on individual channels. Also look at using subgroups so you can manipulate groups of tracks with a single EQ, reverb, etc.
Finally, in answer to your question, I'm sure there's an export audio function in Nuendo. Simply export what you want to bounce together, and then have the resulting file imported back into your project.
Chris
