Podcast from Ian Westermann (knowing 10x more tennis than even a random ttw poster) where he talks swing speeds in answer to subscriber submitted questions:
"..... Next up we've got 3.5 to 4.0 and this is where David falls into play, kind of an intermediate to upper intermediate level and this level of player can start to accelerate a little bit more. I wrote down, and again these general guidelines here, very general guidelines, I wrote down in my notes 80% max. So, you can start to accelerate a little more aggressively when it's appropriate in the point situation, but not more than about 80%, and again 100% meaning that that’s as hard as you can hit the ball.
.........
Then lastly 4.5 to 5.0 level and at this level, consistent acceleration is needed. It's not only something that you want to do, you really have to do it to be competitive and at this point you're taking 90% plus cuts at the ball when it's appropriate. That doesn't mean every single swing yeah you're starting to really take a full cut at the ball. At this level, especially once you get up to 5.0 and above, large chunks of points are now won instead of lost. It's not that a 3.5 or a 4.0 player can't ever hit a good shot and will never hit a winner, you will, but it's a matter of percentages. At a 4.5 and 5.0 level a lot more points are won through forcing errors and hitting winners than at lower levels. So, that means that it's more important to start to really accelerate to keep your opponent from really teeing off and pressuring you over and over and over again.
........."
This "swing full out on every ball for increased control" is ridiculous.