It really depends on the game. The good games are made so that the speed doesn't matter too much.
Metal Black has the slowest ship I've ever seen. Seriously, this thing is pretty much a flying submarine and has the manueverability of a whale. However, it's crazy oversized cannon is as good a defense as you can hope for.
1943: The Battle of Midway and it's remix
Kai start you slow, but your plane speeds up every few levels, otherwise you would get toasted pretty fast.
The
Gradius and
R-Type series start you slow but have speed powerups that can let you go fast. But, they are permanent until you die, there is no speed down.
Take what you've got =P.