I have been using LRT 2 and LR 4, and the time lapses I generate (1080p) are quite smooth. Recently I needed to upgrade the LR 5 and hence LRT 3. Now the videos are very jerky. Is there something I am doing wrong? I have included the option "LRT plus motion blur" in the render.

I'll answer my own question. I made the quality too high and my PC got a bit choked by the bit rate. I used high instead of ridiculously high and it worked ok.