I have couple RB750Gr3 but none of them were able to reach more then 300 Mbps for file transfer in routing only mode (No NAT).
To test this issue try to use the “Bandwidth Test” tool of mikrotik.
Take a look at this post: http://forum.mikrotik.com/t/public-mikrotik-bandwidth-test-server-s-now-shutdown-as-of-april-1st-2025/94863/1
It has ip addresses and usernames for speedtests.
This test will give you a starting point to what you might be able to expect from the Mikrotik device CPU to do for you.
By the way, did you had any chance and looked to the: “Ethernet test results” section of the device?
https://mikrotik.com/product/RB750Gr3#fndtn-testresults
By default I look at the “25 ip filter rules” section to know what is the basic “worst case” scenario for this device.