1) Check if "Hardware" is set to "True". In other case, all redering will be made in Software.
2) Try to optimize the rendering order, i.e. render the same tiles first and then another type of tiles. This also applies to "Figure" type (i.e. render images first, then lines, then rectangles, etc)
3) Try to avoid rendering too much regular lines - since PowerDraw draws them pixel-by-pixel (due to LastPixel issue in Direct3D), it causes additional overhead.

Edit: Also try to match the bit depth of the device with the "native" depth of your video card, sample applies to bit depth of your textures. For instance, there're cards that support hardware acceleration for 16 bit depth only, or 32.
In addition, you might want to modify "MaxVertexSize" constant defined in PowerDraw3.pas, try chaging it to something like 2048 (which normally reduces the performance, but in your case might be different...)