It's just a warning that the programming clock speed is not manually adjustable.
Actually it means that the programming clock speed is not settable over the USB by avrdude.
USBASP firmware (unless modified) always allows a manual clock speed setting to a slow
clock with a jumper. The newer versions of the firmware also allow setting the clock
speed via USB through a new USBASP command.
(There is discussion about this earlier in this thread)
I have no idea what this comment means:
Note: the fischl.de firmware is not made for the Arduino IDE.
I can update sketches or bootloaders using the avrdude commandline or the Arduino GUI
with the latest usbasp f/w.
I use the latest fischel image to allow clock setting from avrdude and I also use a later version of
avrdude than what is supplied by the Arduino team.
(I built the later avrdude myself from the avrdude repository)
The one supplied by the Arduino teams works, but later versions of avrdude have support
to skip over flash areas that don't need to be programmed. When burning a bootloader it
reduces the burn/verify time from a few minutes to 2-3 seconds. It is very noticeable.
I also modified my the fischel f/w so that the leds work slightly differently.
One lights when writing to the AVR and one lights when reading the AVR.
It is kind of neat to see the operation progress but when using the newer avrdude
to burn a bootloader, it is so fast that you don't really see much.
For sketches you have more time to see the operation in progress.