@virgiliomi:
OpenVPN 2.4 adds support for the AES-GCM algorithm, which takes full advantage of the AES-NI hardware acceleration without also requiring the CPU to compute the hash for authentication. Up until OpenVPN 2.4, the only way to use that algorithm with pfSense was IPSEC, I believe. That lets you use your CPU for other functions rather than supporting the VPN connection. (yeah, technically it's all built into the processor, so it's really doing everything anyway, but AES-NI with AES-GCM doesn't affect CPU cycles available for other tasks).
This is mostly not true/confused. AES-GCM is a new cryptographic mode that combines encryption and authentication instead of using a separate algorithm for authentication. (As was historically the case with AES+SHA1 or AES+SHA256 or AES+UMAC, etc.) GCM is dramatically faster than AES-CBC+HMAC on amd/intel architecture CPUs, especially those with the carry-less multiplication operators (PCLMULQDQ, etc.), because it pipelines well. It is not the case that AES-GCM "uses the AES-NI more", it's that the algorithm is simply more efficient on current CPUs. (The catch is that it's either slower or impossible to implement on other kinds of cryptographic accelerators, so it's generally less efficient on older mobile devices or things like intel's quick assist.) AES-GCM doesn't affect CPU cycles for other tasks any differently than AES-CBC except insofar as it may require fewer cycles. (You may be confusing AES-NI with older architectures which used a distinct processor for crypto: in those, you could do other things with the main CPU while the coprocessor was doing crypto.) You generally won't see a dramatic speedup moving OpenVPN to AES-GCM because its architecture prevents the CPU from being able to really crunch on large blocks of data. It'll be a somewhat more efficient (and more secure) option, but it won't work miracles.
FWIW, the lastest intel/amd CPUs include SHA acceleration, so there's hardware acceleration for both encryption and authentication with AES-CBC-SHA1 just as there is with AES-GCM (using AES-NI+PCLMULQDQ). AES-GCM is still faster. The fact that there is a faster cipher mode doesn't make a different cipher mode less accelerated–AES-CBC with AES-NI is still tremendously faster than AES-CBC without AES-NI.
