They can increase arrow speed, but only in conjunction with a few other things.

What a CSS/STS allows you to do is stop your string from oscellating on the shot.Laymens terms, it makes your bow quiet. What that allows you to do is take all the other silencing technology/gadgets off your string. Good bye string leeches, cat whiskers, yarn balls etc etc. With less weight on your string, your bow can propell an arrow faster.

As to increasing accuracy, that kind of depends on you. From a scientific standpoint I don't really know if it does or not. Might help the arrow come off the string a bit cleaner. Thats a bit over my pay grade if you know what I mean.

I havea CSS on my bow. My Iron Mace was a bit loud (it is anyway) so I still have my TRU ball Spiders on, and I didn't lose any speed at all over bear string. You have to fiddle with those spiders a bit, but with just the speed balls, you can acctually pick up speed if you place them right.