Besides the architectural progress, the CDC6600 was impressive for its clock speed of 10 MHz. This may not sound much, but consider that this was a physically very large machine entirely built from discrete resistors and transistors in the early 60ies. Not a single integrated circuit was involved. For comparison, the PDP-8, released in 1965 and also based on discrete logic, had a clock speed of 1.5 MHz. The first IBM PC, released 20 years later, was clocked at less than half the speed of the CDC6600 despite being based on integrated circuits. The high clockrate is even more impressive when comparing it to more recent (hobbyist) attempts to design CPUs with discrete components such as the MT15, the Megaprocessor or the Monster6502. Although these are comparatively small designs based on modern components, none of them get to even a tenth of the CDC6600 clock speed.
Detailed look at the speed of the CDC6600.