Uint8Array over SPI

Yes, by a small amount. But, . . . isn't the data bit rate set by the setup of SPI? (yes)

Setting the baud rate, doesn't seem to matter much.

//byte by byte transfer
data 1 took: 3.80794239044s @20000000 baud
data 2 took: 3.80806064605s @20000000 baud

data 1 took: 4.26346969604s @1000 baud
data 2 took: 4.26357555389s @1000 baud

data 1 took: 4.26337814331s @100 baud
data 2 took: 4.26357364654s @100 baud

data 1 took: 4.22884178161s @software SPI
data 2 took: 4.22821426391s @software SPI

//byte by byte without switching CS
data 1 took: 3.53529644012s @20000000 baud
data 2 took: 3.53545284271s @20000000 baud

There is definitively switching overhead to the CS pin, as mentioned here.

As the image in #16 is helpful here, has the use of an inexpensive logic analyzer been used?

No. Didn't fiddle with that.

What about SPI.send() vs SPI.write() ?

Tested, works, but same speed