What limits ssds from performing well in raid 0?
Not sure exactly. I'd say that it's because the only time RAID really gets to shine is during large sequential transfers and/or high queue depths. But those neither of those are common in most real world use, sequential transfers already get great performance with non RAID SSDs, and if you're transferring from one drive to another the speed will probably be limited by the other drive anyway.
For low queue depth random I/O, the increased overhead associated with RAID is enough erase any benefit from RAID.
I was going to make an external enclosure for a portable drive, that would take advantage of TB3 40 GB/s speed. It seemed that 2 raid 0 ssds would do best there.
As a secondary feature, I was going to use it to mirror drives, if I ever upgrade my PC.
Yeah, but what sort of stuff were you planning on putting on that external drive? Just bulk storage? Games? Would you be doing lots of transfers between internal and external storage? Because none of that stuff would benefit from super fast drives anyway.
Also, TB3 is 40 G
b/s, which is equal to 5 G
B/s. A single 970 Pro is rated for up to 3.5 GB/s. Not that you're ever going to see that during normal real world use though.