Good day,
I would like to ask, I have a hEX RB750Gr3 and I use a fairly simple setup, with default settings: ros 7.16.2, ether1-4 in bridge witch vlan-filtering, several vlan networks, ether5 - wan pppoe,
added l2tp client with ipsec and simple mangle rule that routes some connections to this separate routing table for l2tp, and the maximum speed that I can achieve when working with this l2tp is no more than 50-80 Mbps, I tried replacing it with eoip, but the result is the same, I tried simply sending connections to a separate IP address in the local network, as a separate gateway, and even then the speed does not rise above 80 Mbps. in all cases I observe 40% total load and 99-114% load on cpu3. with the routing rules I have a slightly higher speed, but it is still no more than 100-150 Mbit.

the question is, is 50 Mbps is a practical ceiling for working with monitored connections due to working with only one thread of a weak cpu?

A couple of things mixed together ...
Using PPOE, add to that LT2P with IPSEC AND mangle rules ... all things which are processed by CPU, single core.

I used to have Hex as core router at home.
I had no problem back then to fully use the ISP connection I had at that time (200/20 if I recall correctly) BUT that was using plain ethernet towards ISP modem.
Nothing fancy putting extra load on the cpu.