What is the speed difference between a slapper and a wrist shot? 20mph?
At one time it was probably around that range, but I don't think it's quite that much in game situations today. Even in the hardest-shot competitions, the league's biggest slap-shooters struggle to get the puck moving over 100mph. For example, looking at the 2011 superskills results (the first one that came up on a search), there were four guys -- Fowler, Kopitar, Sharp and Nash -- who couldn't get a slap shot over 95mph. Backes and Seguin were around the 97mph mark. And that's with the puck sitting out on a plate, with time and space to take several strides and a full wind-up and just needing to get close to the net with no goalie.
I'm guessing, without proof, that in actual game situations we rarely see slap shots over 90mph. "Rarely" meaning maybe once or twice a game, among all 40 players participating. Much more often, the shot is compromised by time, space, angle, and the need to put it on net accurately and at a deflectable speed.
In terms of wrist shots, I've seen very little in terms of actual science but I gather that good NHL shooters have wrist/snap shots around 75 mph, and those actually get used with frequency in game situations like odd-man rushes.
All told, I'm thinking we're probably looking at ~10-15mph difference between the styles of shot under normal conditions.
I think the nature of a slap shot makes it hard to read off the stick.
This is true, but the nature of a wrist shot makes it even harder to read. The goaltender and defense have an extra split-second to read the angle and direction of a slap shot due to the windup -- hence the rate at which defenders are able to dive in front of them. Wrist shots don't explode off the stick quite so much, but they are more difficult to anticipate. In terms of reaction time, the suddenness of a wrist shot reduces the amount of information available for the goalie to apply to his muscle response.
I guess it is interesting because you can see thats all the separates good goalies and great goalies and a well placed slap shot has a great chance of going in.
That's the key, though -- placement. In your example of a slap shot from the faceoff dot, against an NHL-sized goalie it's
extremely difficult to hit the exact angle necessary to get past the goalie but still inside the posts from so close-in. This is roughly what they're shooting at from the faceoff dot:
So the only way to get it in the net is to tuck it up under the post, which means an elevation of close to 4 feet in only 20 feet of distance -- that's really difficult to do accurately, let alone under pressure with defenders around. In order to get a do-able angle, you'd have to back away from the net... let's just say 50 feet. A 90mph shot will travel 50 feet to the goalie in about 0.38 seconds.
However, a wrist shot CAN be reliably used to pick corners in tight. So even if we assume that the shooter has to take some heat off his shot from the faceoff dot in order to place it accurately -- let's say the shot is accurate at 60mph -- he's going to get it on net at 0.23 seconds, not to mention the difference in release time.
Basically, this goes back to the point that the only time slapshots make sense is in the case of a long-distance one-timer where the idea is to catch the goalie moving across the crease. Anything else, and the compromise of accuracy isn't worth the extra few mph.