Someone could claim that depends of the time required for flash cell programming. How much is takes ?
If we look into data sheet page 11, tPROG = max. 40 uS.
If we look into IAP section at page 23 the FBUSY bit may not change state for up to 1 uS.
Since I'm expected that ROM Loader uses the same registers FCNTL
5h FDATA
6h like IAP does, I believe that 1 uS is that prog time.
Now, how much time it takes to program 16 bytes ? Yes, you guess 16 uS, regardless the overhead time required by routine programming which ends waiting for FBUSY to return to 1.
Included in this routine are preprogramming assessment and postprogramming verification, too.
How much time it takes to send serially the payload 16 bytes at 115k baud rate ?
20 bytes x 11 bits (Start bit + 8 + Parity + Stop bit) x 1/115200 = 2 mS which is by far greater than time required to program them in flash.
Thus we can't say here it's the bottle neck which limits the speed programming.