Exactly ahill. When specifically mentioned that they will not increase their power, then it's only speed. If only speed is increased, assume both.
The reason speed is talked about in the cases of Freeza and Cell is because speed is the issue. You guys have to start paying attention to the context better.
Vegeta is bragging about keeping up with Freeza's speed. Freeza says ok, let me increase my speed then. If he said I'll power up, it doesn't have the same effect.
Cell is getting made a fool of because he's so slow. He tells Gohan he wills increase his speed because his speed is the issue at the time. Clearly if Cell and Gohan's speed and power are balanced and Gohan can avoid him that easily, what the fuck can Cell's power to to Gohan's? Not much.
Put it this way, for those who can't keep up.
Cell
~ Speed 75
~ Power 75
Gohan
~ Speed 100
~ Power 100
Ok Cell can't land a hit, Cell increases speed.
Cell
~ Speed 100
~ Power 75
You do realize that even though he's as fast as Gohan now, Cell's punches are going to do jack shit. He would be too weak. BOOM!