Yea, with the kinds of low-CPU plugins you're using (not to mention, no virtual instruments), you shouldn't be having drive IO issues, unless you're on an old system with non-SSD drives. Even then, it's a little surprising. If you have a fast USB3 or Thunderbolt external hard drive, maybe try moving some of your source files there so you can split the load. Beyond that, I'm not sure what might be causing it.