Are you using P3D?
I am using FSX-SE and also experienced low frame rates with the 757.
Now I am getting good frame rates again (35-40).
What I did was, I removed cabin an wings from the ACE.

I had already tried this before and it did not work because I only removed from one plane which I was using.

I then went and removed the cabin views from ALL my 757 downloaded liveries and that appears to have done the trick.