Greetings,
You want your I/O buffer to be as low as possible when tracking. 128 seems to be a sweet spot for acceptable latency. Lower is better if your system can function without pops and other audio artifacts.
This will really depend upon whether you're tracking only or mostly audio vs. using many software instruments.
Having separate external hard drives for sample libraries and for project files will also optimize performance vs running OS/apps, sampler instruments and projects from the internal drive. The internal drive is ok if your projects are smaller, but you'll really need the external drives as track count increases.
Keep an eye on the Load Meters (for cpu and hard drive stress) in the transport. You can double-click it to open it in a separate window.
For mixing with lots of plug-ins, increase the I/O to 1024 to reduce cpu stress. At this point, latency is not a concern.
I set Plug-in latency Compensation to All, and turn off Low Latency mode.