Depends on a few different things:
What do you have him equipped with?
If Krelos doesn’t have to hang around to keep hitting towers, then speed is a priority.
Who are you hitting?
If you’re hitting up, or even just hitting a strong base, Krelos will need more time over the target to do his damage.
Is the target defended?
Against a defended target, Krelos can more easily speed past a base without killing it, as storm/earth shields may block his attacks on red mages, and red mages may block his other attacks. Plus, of course, shield and hammer consumables.
How good is your timing?
I’ve done a lot of Krelos runs since I got him, and I’m still working to get his timing down just right. Things like supershots can throw you off, and if you use one of his abilities too early, you may find out that you can’t chain them as neatly as you’d like.
What are you trying to accomplish?
I find he’s most useful for hitting undefended enemy bases, countersniping, XP runs, and gold runs. He can easily be timed right for invader bases, making gold and XP runs a breeze; for those you can use all the speed you can get. For attacking and countersniping, speed is also key - and with countersniping in particular, you often don’t even try to get 100%, so it doesn’t matter if you don’t get everything perfectly.
Basically, he doesn’t have the best spellset for attacking tough, defended bases anyway, so if you’re not going to rune him for high-speed smashing, you’re probably missing the point of him.
Edit: Oh, you’re using him for invader bases? Yeah, you can totally just time his spells for that. It helps to remember that both his Speed Demon and his Take That spells will hit towers both in front of him and behind him.