Nice looking interface a lot of features for a great price!
I’m not too sure how performance is with Behringer stuff myself. Start off the lowest sample rate anyway but make sure it matches the engine sample rate and 256 block size maybe, seems do okay for me (2nd scarlet 2i4).
Having a dedicated interface can take some of the audio load off a cpu. Using a low buffer/high sample rate will increase cpu. A higher buffer will decrease cpu load as will a low sample rate. You would have to find a good balance. Just remember each time you change ASIO settings you might not necessarily see a difference until you restart Rack and or reboot. ASIO doesn’t take too kindly to having settings changed on the fly sometimes, from my experience anyway.
Make sure you have the correct/latest drivers and if it is a latest driver perhaps a previous driver gets better performance?
On the RCA outputs. You could look into getting a couple good quality 1/4" Jack to RCA adaptors or a TRS to RCA cable it would be on a balanced output, RCA does not have a Ground though so probably a waste of money if it is going to a Hi-Fi.