Edirol UA25, macbook pro, Cubase

Hi, I'm struggling with a ua25 I just bought.
I was told to you guys may be able to help me out. :]
 
I'm using it with Cubase 5 on an early 2011 Macbook Pro running Yosemite 10.11.4
I set it up as per the manual, got loads of HD space, but I keep getting stuttering when my projects get a little busy.
It gets even worse when streaming audio/video.
 
Any tips?
 
Thank you!
Lee