So I have some further observations to share, maybe it helps.
It seems that the aborting stage of current preview renderings, be it OpenGL or RTP is causing problems - not always, though. Sometimes TG will hang a bit during this stage and since it seems to abort any current calculations when you save it might be what's causing the lags, not the saving or accessing files or network.
Then, TG hangs for me when I try to open from library. I have to test this one a bit more but so far it just hangs for minutes until I kill the tgd.exe process.