What version of code is the MX67 running? I had found a major VPN bug in 14.37 and older for the MX67/68. My MX65 would do 120/12 all day long for tunnel traffic, swap for the MX68 and it barfed.
Any easy test is to enable split tunnel so internet speed tests are local, not over the tunnel. If you see improvements you know you are hitting the same bug, as the issue exists only the tunnel.