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Keyword Abbreviation Token (hex) Version(s) Classification
SLOW SL{Shift+O} FE 26 7.0 Command and Statement

Machine state managment.  Directs the CPU to operate at "standard" speed.

SLOW sets the computer to operate at its default ("compatible") speed.  On the C128, this is about 1MHz and programs will run at about the same speed as on the C64 (actually a bit slower).  In this mode, the VIC-II (40-Column display) is fully functional. 
Note that any SPRITEs that are enabled will further slow down the CPU a bit (generally two CPU cycles for every raster of each active sprite). 
The system "Jiffy Clock" updates at the normal rate of 60 units ("jiffies") per second (see TI/TI$), regardless of FAST or SLOW mode.  The SLEEP statement uses the Jiffy Clock so it is not affected, but timing delays using FOR/NEXT will execute twice as slowly (as compared to FAST mode).
Attempting to supply a parameter to SLOW will generate SYNTAX ERROR.
Technical Note (really outside of BASIC): The KERNAL will set the CPU to SLOW mode when it performs external I/O (any device except keyboard or screen), and then restore the previous speed upon completion.
On the C64, some programs were written that would blank the VIC-II screen to increase speed.  Later the screen would be restored to show results.  That method improves speed by about 10%, but FAST improves speed by about 100% -- a big difference.  SLOW is the only possibility on a real C64, but using POKE 53296,0 will restore 1MHz speed on a C128 that is in 64-mode (this only makes sense if it was previously POKE'd into FAST mode).
SLOW : FOR T = 1 TO 1000 : NEXT
(delay about 1.4 seconds)

FAST : FOR T = 1 TO 1000 : NEXT
(delay about 0.7 seconds)

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