The new ipad has a lot more pixels in the display, so displaying a raster map at that resolution is expected to be slower.
If you have a large number of routes, or routes with a large number of points on the display, that has a big effect on drawing speed. Save your data, then I suggest deleting all the overlays except the one you are working on.
I have used MM since 2005. I have learnt to hide the track, not have too many waypoints etc... all to stop my pda grinding to a halt. I have no overlay whatsoever on my ipad and it is still jerky to the point of I don't want to use it. Comments made like Richards above just make you think, why not give Viewranger a go. I'd never heard of it before reading this post but now I think why not give it a go.