I upgraded a ~5 year old CX to 5 last week (from 4.1 mind you), and noticed that the boot time was really blown out. A good 25 minutes for the system to come up and be ready (with the PRI trunk ready to go).
Response when jumping through pages in ESM wasn't too bad, but a bit slow, but at the time I put that down to the AV scanner on my laptop or something and didn't worry too much.