the slowest speed for dvd +R is 2.4x, 1x can't be done.
the slowest speed for dvd -R for recent firmwares is 2x even if its a 1x disc.
so 2x and 2.4x are the slowest you can have. These speeds should be fine for everything though.
If you have problems with movies playing back properly, use different (better) discs, not slower speed.