AlwaysOn AG - CPU limitations
This is documented here on BOL: Thread Usage By Availability Groups.  For redundancy purposes, here is the breakdown:


3 to 10 shared threads (but can increase depending on primary replica workload)
(primary replica) 1 log capture thread per primary database
(primary replica) 1 log send thread per secondary database
(secondary replica) 1 redo thread for each secondary database
(secondary replica) during a backup, holds a thread on the primary replica for the duration of the backup


Other points to consider:


There is a shared pool of threads for hadr, and this is the max number of threads except 40 (so hadr worker thread pool could take a significant amount of threads)
Idle threads (about 15 seconds of idle-ness) can be released back to the pool


Other references:

AlwaysON - HADRON Learning Series: Worker Pool Usage for HADRON Enabled Databases

Monitoring SQL Server 2012 AlwaysOn Availability Groups Worker Thread Consumption