The CPU monitor displays very high usage even with only one layer ?
Modul8 uses some threads that run very fast especially when Quicktime is used to decompress frames. When the CPU monitor shows full usage, it does not mean at all that Modul8 reached its speed limit. It simply means that Modul8 uses all your CPU power. However Modul8 may re-arrange its internal activity when new layers/movies/effects are added. In other words, do not use the CPU monitor to see if Modul8 reached its speed limit.