Just from what I notice in-game.
I haven't noticed any difference between what melee attacks I can block/counter swing speed between a +3 buckler (106 speed) and an unloomed steel shield(61 speed). I'm not sure there are any situations where high shield speed is important, unless your ping is bad enough where you have less than 1/5th to 1/10th of a second to react.
All I've noticed from trying different shields are weight (crushthrough, movement speed), ability to block ranged attacks (size, width for multiple ranged, height for single), crossbow penetration resistance (resistance, mostly not a problem past 20 if you're not wearing cloth) and overall durability vs regular weapons and bonus vs shield weapons (hp/resistance).
There are some cosmetic things like long-ish shields and larger shields don't look like they hit with nudges properly because the range never changes, and some shields are far less ugly/clunky for use on back (hide covered shield vs leather covered shield for instance), but those can be managed by changing your models.
I still have no idea how bucklers function while sheathed, or if shield size plays any factor in back shield hits, or if it's really true that couches count as "missiles", but I'm sure those are easy things to test for people who have time/willing partners.