Intel ark suggests that's a perfectly reasonable processor. It's a few generations behind but has a high clock and encryption acceleration in its favour.
Things to check including that you observe the high clock rate and boost rate as you are testing your VPN speed.
Online sources suggest an overhead of 10% to 15% is reasonable.
One source talks through things you've probably already considered but dropping the encryption level should be a good sanity check. Unfortunately their speed comparison link is broken but there seem to be other useful things on their main page.
