Depends on whether you're searching for bc1 or 1 address. bc1 is not case sensitive which greatly contributes to faster times since the program does not have to consider every combination of uppercase and lowercase letter, but a normal 1 search is case sensitive.

Considering that 4-5 characters (+ case sensitive) can be found almost instantly, it's not surprising that 50% of 6-char search space is exhausted in a few days on old hardware. Newer computers have faster cores with more threads and can search the same 50% of space in several hours.