So, I need a bit more performance from this. The CPU frequency setting offers choices of:
500
550
600
650
700
750mhz

Is it safe to run this at 750mhz?

You can read the warning message of changing CPU or memory frequencies here https://wiki.mikrotik.com/wiki/Manual:R ... s#Settings
CRS109 is a switch, if you use it as a router then the performance will not be as good as if you were using a Router model...