There is no official number for "normal" CPU. Some things are memory intensive, others are CPU intensive. 4500 simultaneous connections is fairly low. On the 3650 only the SSL handshakes are done in hardware. You should take a look at the number of requests per second. Easy way is to clear cluster stats and "show server stats 1". One of the columns is Total Requests, you can see the difference between each entry. 22% CPU is very low. DX can run fine well into the 90's% with no issue. Performance for all users will be the same until CPU reaches 100%. The great thing about DX is ActiveN will allow you to scale linearly across N boxes if you need the capacity.
This is not an exhaustive list...
Uses Memory:
Connection count
SLB
ActiveN
FWLB
Uses CPU:
Cluster transactions
AppRules
New SSL handshakes in xx00 boxes
Bulk encryption in xx50 boxes
New Proxy connections (cluster/forwarder)
DJ Skillman
Manager, Technical Marketing
DX Application Acceleration and Load Balancing