Both routers support AES hw acceleration. So both can do way more than 70mbit/s
with encryption.
Also, AFAIK, increasing the CPU cores, will not increase your throughput if all traffic flows through a single IPsec session.
EoIP and L2TP can both do hundreds of mbits/s without breaking a sweat. So, it doesn't really matter which one is most CPU efficient. Your bottleneck is most likely elsewhere anyway.
Post the output of /tool profile cpu=total duration=5 while you have high CPU usage to see what exactly is taking up all the CPU.
Encryption shouldn't have used the CPU in your case, since both routers support hw acceleration.
Unless you have some weird IPsec configuration that doesn't use the proper encryption algorithms.
Post your full configuration from both routers (/export hide-sensitive) so we can help further.
Also, I am not sure if FastTrack will help in any way here.
EoIP is proto 47 (GRE), not 6 (TCP) or 17 (UDP) which FastTrack only supports - according to the documentation:
https://wiki.mikrotik.com/wiki/Manual:IP/Fasttrack