Possible bottleneck w/ SATAI drives for OS?

As stated in my previous post, my h/w is as follows:

Motherboard: ASRock Rack EP2C602-4L/D16 (4 Intel GbE interfaces)
Processors (x2): Xeon E5-2670 - 8 cores (HT) @ 2600MHz
Memory (256GB): Kingston 16GB 2RX4 PC3-12800R DDR3 Reg ECC Server Memory SL16D316R11D4 (x16)
Storage1 (120GB ZFS): Samsung EVO 850 SSD SATA 6Gbps <---- SLOG for ZIL offloading
Storage2 (24 TB RAIDZ1): Seagate IronWolf 8TB 7200RPM SATA 6Gbs (X4)

For primary OS HD, instead of a USB stick, I am considering using two (RAID1) 74GB WD Raptors. My worry is that their max speed of 1.5 Gbps will bottleneck the system.

Is my concern warranted?

With FreeNAS, once the OS is loaded it doesn’t do much writing to the OS drive, that is why I just have my FreeNAS systems loaded on USB thumb drives.

Is that a benefit unique to FreeNAS?

There are a few OS`s that can use this “feature” if you will. The truth is it just doesn’t take that much to run a NAS, And all the Plugins and Jails are run on the pool. FreeNas Was not built to do this so much as it’s just how it is.