These are invoked in the 'mode' range of 400 to 499.

The cost of async vs. sync for the software algorithm implementations
varies.  It can be as low as 16 cycles but as much as a couple hundred.

Here two runs of md5 testing, async then sync:

testing speed of async md5
test  0 (   16 byte blocks,   16 bytes per update,   1 updates):   2448 cycles/operation,  153 cycles/byte
test  1 (   64 byte blocks,   16 bytes per update,   4 updates):   4992 cycles/operation,   78 cycles/byte
test  2 (   64 byte blocks,   64 bytes per update,   1 updates):   3808 cycles/operation,   59 cycles/byte
test  3 (  256 byte blocks,   16 bytes per update,  16 updates):  14000 cycles/operation,   54 cycles/byte
test  4 (  256 byte blocks,   64 bytes per update,   4 updates):   8480 cycles/operation,   33 cycles/byte
test  5 (  256 byte blocks,  256 bytes per update,   1 updates):   7280 cycles/operation,   28 cycles/byte
test  6 ( 1024 byte blocks,   16 bytes per update,  64 ...