I'm using Reaper to create backing tracks for a band I am doing with my brother. I have a Focusrite 2i4 interface.
So far I have only recorded guitar. Even with that I cannot notice the latency.
In Reaper you need to set it up to use the Presonus device and driver.
First go to Preferences -> Audio -> Midi Devices. In there find your Presonus device, right click and Enable it .This screen show shows you were you need to be rather than your specific audio device.
I'm not at home so I can't do a screen shot of my 2i4 setting in reaper but this next shot show you were you need to go next.
In my case I have the ASIO Driver set the the Focusrite ASIO driver. Then under that you pick which inputs you are using.
Does this help?