I have just checked https://en.wikipedia.org/wiki/Rugg/Feldman_benchmarks (thanks to drogon for this link). It shows that the SWTPC 6800 Basic is 5-10 slower than Apple II Basic which makes more plausible the claim that the 6502 performance is 2-4 faster than the 6800. The data for the Altair 680 confirm this conclusion too.
It is also surprising for me that the IBM 5100, which cost above $10000 in 1977, was 3-5 times slower than the Apple II.
6502.org
The 6502 Microprocessor Resource