The XR18 has no dedicated audio drivers for OSX and uses it's Core Audio.
But Core Audio in Yosemite was so flawed, that some people downgraded to Mavericks ... I found such reports about OSX 10.10.2 in combination with Logic.
I don't know, if it was completely fixed in 10.10.5 ... maybe upgrade to 10.10.6, which was the most stable Yosemite version from my personal experience.
I remember, that I had similar issues with my old Tascam US-122MkII, when I got my Maccbook Pro with OSX 10.10.3 ... Core Audio was a mess and only Tascam's proprietary driver worked without latency issues, though Core Audio was advertized as a working alternative.
I never tried Core Audio again and currently use 10.11.6 ... maybe I should try Core Audio again out of curiosity