Any ideas here how to make it return faster?
I’ve stripped it down and it’s really basic..

Lever on bars or under seat?
I find slow returns are normally due to not being able to get full range of movement at the post
On an under seat style I found after a heavy landing my saddle nose down stance and I assume squished rails, the saddle interfered with the lever.
On bar remotes, non stealth, it’s usually down to cable routing fouling on mast compression

I’ve taken the cable off the post and just compressing the seatpost then releasing the lever manually.
It’s really slow and almost stops. Can’t see an air valve so I assume the air pressure is fixed.
It’s less than 12 months old so I’ll get it back to the shop.

Sounds like the cartridge has gone?

Echoing above, either your cartridge is gone. Or the guide bush isn’t entirely sitting flush with the aluminium sleeve it sits in. Especially if you’ve had it to bits recently, that’s what my friend found… 🙄