It seems like nowadays you can have slows for days, but if a champion has a semi-decent form of mobility, it just won't matter a whole lot.
EDIT: Non-target dashes. Examples 
Dash 
Tumble 
Spin 
Dash.
NOT examples 
Ult 
W 
Ult 
Wall grab
Whether this be slower completion times (E.G. longer time to complete targeted dashes)
True Damage God1 voteswhat about if they also did animations? like slowing down an anivia q with nasus wither or something x3
IcyPepper1 votesThat's kind of intentional. Now, hear me out, there's two reasons why I disagree but I'm going to try my best to give a decent explaination:
Slowing down animations might make it look awkward. Having it affect range instead of speed would make it awkward and possibly(?) hard to program, especially for gap closers like Akali's R or Ekko's E. And don't even get me started on Zed.
Hard cc is more effective against mobile carries, and soft cc is more effective against immobile tanks/bruisers. Why? Hard cc allows you to blow up a glass cannon that's out of position, but a tank is still going to shrug off that damage. Rotating cc in a teamfight is an important skill to have, using your soft cc on the frontline is better than the mobile carries not just because those carries can dash and whatnot, but because you have less tools to keep the enemy. Keeping soft cc less effective on mobile carries will encourage people to use it on frontliners, and encourage them not to waste hard cc on tankier champions.
However, your idea isn't absolutely terrible or anything. It'd be cool if there was a form of mobility that was affected by movement speed, both positively and negatively.
venomous frost1 votesOP is talking about dashes, neither akali's R, ekko's E or zed's W are a dash. (well maybe akali is, but it's targeted, and i'm assuming OP has untargeted dashes in mind like graves)
EDIT: ekko's first cast on E is a dash.
IcyPepper1 votesI tried to cover both, and I specifically called Akali's and Ekko's gap closers just in case.
For the rest, the animation range or speed would be weird if it was affected. That's basically my tl;dr
LatetotheRace1 votesFeasibly, either the slow would simply make the ability take longer to complete (longer animations) or would actually reduce the range that the ability would cover. The reason I think this would be beneficial, is that champions with slows only, have no real counterplay against incredibly mobile champions which are dominating the meta. This would provide them with a semi-decent option to really do something worthwhile.