All performance issue requires the same troubleshooting steps.
1---capture a trace going directly to the server.
2--- get a show tech on ace and call it before_error_show_tech
3--- capture a sniffer trace going to the same url but via ACE
4--- get a 2nd show tech and call it after_error_show_tech
Now, you need to compare the 2 show tech and see where there is high inter-packet delay.
Check which counters increased between the 2 show tech.
If you can't find a solution, send the info to the TAC.
Gilles