I would not do that. Sure the cutter might still be quick even with the speed decrease, but then you have an average speed weak sword. The cutter is very weak sword that makes up for its weakness with quick hits.

IMO plate mail is pretty much worthless. It's only function is to keep new recruits who are struggling alive in Tier 1. Also, it only offers normal protection until you hit the 5* versions. You are going to want Elemental/Piercing/Shadow protection for T2 & T3.

I tried out Spiral Plate and couldn't believe how slow it made me feel. I'm used to running around with VH to Max ASI, so after removing all my equips and donning Spiral Plate I was suddenly at Medium ASD using a cutter. It was painfully slow, I was missing mobs, getting hit, couldn't dodge or shield in time... terrible.

You usually can't tell ASI differences by looking at someone else in the game, that has been well established before. The differences are small enough that they are not comparatively noticeable. Screen recording and using a timer shows the speed differences though. Even so, the changes in speed makes a big difference for the knight in terms of responsiveness when in control.

As for the cutter line... it is a great set of swords for T1 and most of T2. But going deeper is a lot easier with weapons that have more knockback and sooner interruption. I've seen people successively use a Dread Venom Striker as their primary weapon all the way to the Core but it requires an exceptional amount of skill compared to the other sword choices. If you really love the cutter then maybe you are up to the challenge :) Top notch for Jelly King though.

OK, lets pretend that the speed nerf is imperceptible (which it isn't, but we are pretending), plate mail is still crappy armor. I'll repeat. Only Normal defense until you hit *5. Only normal defense is fine for Tier 1 (because it is only normal damage), but prepare to get shredded in T2/T3 if you have been stomping through T1 relying on high normal defense. Why take an attack nerf for that? & good luck using a DA/GF (you are sure to notice the speed hit then). Also, remember that attack speed has a direct effect on how long it takes to bring up your shield after an attack.

I will say this again too. Plate Mail doesn't complement the cutter series well. The cutter would benefit most from DMG bonus because it is a fast weapon that deals low damage. Culture is right too about the Cutter line in T3. It is really rare to find somebody that still favors them at that level.

I will sum up the short answers.

"Should I get wolver or spiral plate for general use? Alongside cutter series weapons."

Wolver.

"And how noticeable is that attack speed decrease?"

Attack speed +/- 2 is not so noticable for fast weapons, but makes a huge difference for slow ones.

(Also, high ASI is even nice on fast weapons, though a much lower priority then DMG +)