Optimisation Blocks or Sprites

Hi All,
I have a level with many mesh houses, to increase the frame rate I have some of the backgrounds constructed of oblong
pcx sprites with house images on.

Is it faster to have Sprites, or should I have Blocks with images on. My ingame frame rate is 60 to 70 fps at the moment,
but there is a small lag in someparts of the level.

Many Thanks

Sprites are faster because they are a single polygon, blocks are at least 6 polygons.