As you may already know, USBasp is a free open-source USB-based ISP programmer for Atmel"¹s AVR microcontrollers.
It uses a high speed clock for programming so AVRs with clock frequencies lower than 1.5MHz can not be programmed unless USBasp lowers its clock.
This decrease in speed can be done by a jumper connected to PC2 which its designator in schematic is JP3(Slow SCK).Automatic Software Controlled Clock Jumper Free USBasp ATmega8 2011-05-28
So for every new AVR which has an internal 1MHz RC oscillator the jumper must be set and after fuse bit configuration the jumper must reset for high speed operation.
This may be annoying and confusing for some people and I don't know the reason of using this way.
I have changed the code of USBasp for automatic software controlled SCK option to support targets with low clock speed(< 1.5MHz) and there's no need to JP3(Slow SCK Jumper) anymore!
Just download the HEX file and program it to a NEW ATmega8, config fuses with HFUSE=0xC9 LFUSE=0xEF, and check it!
If you find any bugs please report it!