How to keyframe speed of particles?

I'm trying to make particles decelerate until they stop, but can't get it to work properly.

I've tried Keyframing the particle's velocity "Normal" option, but that only affects new particles that are generated, not already existing ones.

I've also thought about just making each particle into an actual object of which the location can be keyframed, but don't think that'd be a good idea considering the amount of particles.

Maybe there is a good way of doing this with nodes, but I believe you can only see and use information of a particle, not change it (correct me if I'm wrong).

I can't find a setting that affects all particles at once. Is there a convenient way of doing this or do I have to resort to just rendering the particles moving very slowly for like 2000 frames and then quadratically slow down the video? (If I just slow down the video with like 100 frames, it wouldn't be smooth at the end)

You can animate the timestep. A value of 0 stops the animation, higher values increase it. In this way you can change the speed at any keyframe of any particle system.