Suggest a model for medium office site-to-site IPSEC tunnel

Hello, I need to implement a site-to-site VPN tunnel solution, and wonder what model to choose. In my previous project I used RB2011UiAS-2HnD, for a small office, but at high tunnel load CPU usage hits 80-85 present, which is no good, internet connection suffers significantly. I struggle with performance of different router models. What model would you suggest to connect two offices with 10-20 employees with a lot of traffic between the offices. Thanks.

It would probably help people to respond if you put some numbers on the question since “a lot” is a relative term which is dependent on your perspective!

e.g.

What speeds are the actual WAN connections at each site?
What percentage of the traffic is between the two sites rather than internet access?