Define very slow so we have something to compare against.

Code:
outvar.0 = invar.0
outvar.1 = invar.0
outvar.2 = invar.1
outvar.3 = invar.1
outvar.4 = invar.2
outvar.5 = invar.2
outvar.6 = invar.3
outvar.7 = invar.3
outvar.8 = invar.4
outvar.9 = invar.4
outvar.10 = invar.5
outvar.11 = invar.5
outvar.12 = invar.6
outvar.13 = invar.6
outvar.14 = invar.7
outvar.15 = invar.7
The above brute force version takes <4.1us @64MHz. Extrapolating to 4MHz = 66us or 15150 times per second.
I don't think I can come up with a faster PBP version, perhaps some ASM guru can but knowing you you don't want that :-)