"FS64.SDA", a Commodore-64 file that self-extracts into two BASIC
programs.  

Load and run "FS64DOX.BIN" to see how a simple modification wll let
your C-64 use Fast Serial mode when using either a CBM 1571 or a CBM
1581 disk drive.  The mod consists of installing two jumpers in the
computer; just four solder connections.  You will then be able to use
both Slow Serial mode and Fast Serial mode (with devices that support
it).  

"FS64.BIN" demonstrates one use of Fast Serial mode.  Load and run the
program to install a Fast Serial LOAD routine.  When you do a BASIC
load (in direct mode), Fast Serial will be used if the drive supports
it.  You will get a substantial speed increase.