The TL;DR Version:
At the core of the matter -- slow magic favours preparation while fast magic rewards improvisation.
Thus, for slow magic to be useful, there needs to be a way that this preparation can defeat the more rapid-fire style casting of the fast mages. Or a flaw in the fast casting system that can be reasonably exploited.
The MWoT Version
On Action and Reaction
First thing, slow magic might be slow to cast, but that does not mean that people are necessarily slow to react. In fact, I would think that they would actually be a bit proactive at sniffing out the evil mages since the more time they have to prepare, the better their chances of victory are. This would not necessarily be an inquisition or zealous crusade, but can take the form of a more observant guard or court mage using a detection ritual.
Magic detection spells to detect the traces of demon magic might be a developed thing, creating pockets of land where evil mages will almost certainly be detected fast.
Special Weapons and Tactics
In terms of actual combat, I can see two main categories where slow magic will be a favoured tactic
- Pre-combat enhancement of the mage-killer strike force
- Tactical pre-emptive bombing of the area that an evil mage is found in
Second, as written, there will almost never be a pure caster fight -- the slow caster would almost certainly lose every time in an impromptu fight to all but the most inept evil mages. That or the evil mages that don't realize that they are actually evil mages.
Instead, the more likely thing will be fighters enhanced by slow magic rituals to be able to combat an evil mage -- these can be two different people or one single eldritch knight slash kung fu wizard. Slow magic might also be used to set up a zone that favours their side and/or hinders the evil mage.
There is much in the question about how the casting of slow and fast magics work, but there is nothing about what each branch of magic is particularly good at. Those differences in specialty could be crucial to how a combat goes.
Slow magic can possess the advantage of versatility in function -- the runes and geometries and materials of the ritual become essentially a language and with it, a large amount of spellcraft can be written and brought to bear when needed. This will likely mean that given enough time and study, there is a spell for every situation, though it might take time to find that spell.
In contrast, fast magic might be really good at a couple of things depending on its caster, but not be really adept at much else outside of that. These things can even vary from caster to caster, giving every fast caster a specialty but also lacking the versatility that slow casters enjoy. Given the possible lack of a formal education for fast casting, learning new spells in the fast system might be inconsistent and patchy.
As one example, slow magic might have a myriad of spells to handle fire all with slightly different parameters that require different spell circles. Fast magic might only have one to three spells to do the same based on power, but it relies on the will of the caster for everything else.