1.3.2 runs smooth, but starting from at least 2 snapshots before the 1.4 pre-release I've had a hard time playing some superflat maps.

Normal maps play fine, no more lag than usual and no eventual crawl. Some of the presets for superflat also have no issue.

Try the 'Overworld' preset though, and you may experience lag. The lag leads to eventual crawl which leads to items not being placed in chests or your inventory properly.

I will try different combinations of features when I have more time and report which features are causing the lag.

Let me be clear again - 1.3 ran fine on superflat, the normal superflat runs fine on my 1.4.x, but it is certain features of other presets and custom maps that is causing the lag. My computer runs MC fine 99% of the time.