you want fast path, interleaved does packet analsys, it looks it everything to make sure there are no errors but it slows down your internet because it takes time to look at the packets. fast path doesnt look at anything it just passes it throught he dslam and your ping times will probally be cut in 1/2 if not more.
tell them to switch you to fast path.
are you sure they did not switch you back to 1.5.