I have a 15 minute Greenscreen project with a talking head, and sometimes body too.

The idea is that the talker is filmed so that I can put a plain backdrop behind him, and key words appear over his left shoulder.

15 mins seems an awful long time in AE, with all sorts of problems with audio rendering even with short work areas etc...

How would you handle this?

I've put the greenscreen guy in a precomp, and that seems to help, but its so hard doing previews and syncing..

would I do better to export the green part as a QT with alpha (or something different?), and do the words in the background in premiere?

Lower your preview quality while sync-ing: that should speed the video render up closer to real time. Or make a 'proxy' video file at a low-res/high-compression, and include it as a guide layer. AE is not the 'right' place to do your audio work...but if this is the only problem you're having, then streamlining the video pipeline might get you what you need.

edit-
Pre-rendering the keyed footage-- which a technical way of suggesting that you make the "QT with alpha," file you mention-- would speed up the AE or Ppro preview-rendering considerably. Or disable the keyer temporarily while synching in AE or Ppro.

if you have Prem Pro & the Suite with dynamic link

edit in Prem Pro. pull a temp key, place graphics & text

dynamic link -> make new AE comp

the copy and paste your TL into AE, or import the Prem Pro proj into AE.

pull the key last. also, since Prem pro supports TGA sequnces, you could do the key first, output to TGA seq and import that into Prem Pro and then add text ^ graphics from there. PP can do most of what AE can, and play it back.