[racket] base64 decoding

Has anyone written a faster Base64 decode than in the "net/base64" module?

("base64-decode-stream" is suffering when tens of MB are run through 
it.  Perhaps due in part to whatever overhead the input port has. The 
code looks pretty good.  I'd probably have to try using block reads to 
reduce calls for the port abstractions, and also make a variant that 
works directly on byte strings without ports.  Just checking whether 
someone has already done this.)

Neil V.