The trick is to find the sweet spot for your drive/media/firmware combination. Run a SurfaceScan check of your disc to make sure you have clean sectors (In NeroCDDVD2000, on the Extras menu click ScanDisk, other utilities may have similar features). Too slow or too fast can make irregular pits and lands with some environments. Typically, slower burns can result in slower reads when it comes to DVD's. The usual rule of thumb is to not go less than 1/2 the rated speed of your media, and not more than 2X the rated speed if you are running a hacked firmware. Just try different combinations until you find the one that fits.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.