Shield user doesn't even need more kills to be effective.
You can save a lot of teammates by catching ranged fire or taking pressure of your allies. If I'm fighting with a good 2h vs 2 enemies I can just try to get the attention of the most dangerous enemy, shield means I can focus on staying alive really easily, he can kill the easier one and then we backstab the other. Yeah, isn't visible on my score sheets, but it helps win rounds. Often the dangerous enemy for the 2h is also a ranged player, so it can get a few cheap kills. So as tactical tool shield beat 2h by a long way imo.
For individual fights vs 2h swing speed is entirely not a problem. I can swing just as fast, unless he has a much higher level than me. What is problematic is running tricks like running to the side which adds to his speed or running a backwards circle so he is out of my range on my turn and can swing without blocking again. Or feint faster. Yeah, shield slows you down. Simply means you have to change your fighting style. Be cautious with attacks, as long as you have your shield, it is simply your mistake if you make an attack and he hits you first. If you attack on the right opportunity with the right attack you will hit first, and many 2h can't block it. If they do, well they will focus on defense more and you have more leeway to focus on attack make feints etc. Always try to keep the pressure up, though. Even best 2h will be in trouble if they don't carry anti shield weapon. Yep, this fighting style means your shield breaks faster, but it's just like ammo for archers, if it allows you to kill 2 enemies (or stall a very dangerous foe for the round) before you are only 1h fighter it was money well spent.