Hei Alexander, en Oslo-boer hilser
What is "fast enough" depends on a lot of things. How long shutter time can you use (both how steady your hands are and how still the subject is)? How high ISO is acceptable in any given situation?
The 35/2.4 is faster than the 18-135, but not by
that much. It might be enough, though. Say, if you have to use ISO 6400 to get a blur-free picture with the zoom, you might get away with 3200 or maybe even 1600 with the 35. Sometimes, though, there is no other way than using s flash or a tripod.
That said, the KS-2 is very good at high ISO, and cleans up quite well with a good photo editor.
I find the 35 to be good for use indoors if the room isn't too small.