How can I figure out what made my Fitter (Place and Route) time balloon?

Thanks!

Partitioning is a great idea. One other thing I did was look through the timing report to see if anything stuck out. Sure enough, the were a lot of address lines going to a memory that were (apparently) hard to place in order to make timing. Adding an extra register stage seems to have helped and I'm going to look for more ways to reduce the combinational logic as TCWORLD suggested.

To answer sstrell's question, I am working on an Intel i7 (~3GHz) with 12GB of RAM. Yes, I know I should get some more RAM too.