Shure there are many things to consider when comparing performance, but...
Well, I've tested several bigger MDBs and opened them without changes in Access
2007 and compared performance to that under A2003. In every case they were
significantly slower. Switching between records is much slower.
And if I converted those MDBs to ACCDB format, performance decreased even more.
Opening a form with 10 thousands of underlying records sometimes takes to 10
seconds.
This behaviour has meantime been confirmed by many other users.
Ciao, Sascha