My fleet of starships usually is fast. Rarely do I have slow ships. Even my battleships usually carry two engines. The only exception to this rule are defender type starships, and dreadnaughts (huge class ships with 1 engine). I've fought humungous wars where I only won because of sheer technological edge and speed. In nearly every battle, I was outnumbered 3 to 1, and was fighting three different nations at once. Had I not the speed I had, especially on my strikefighters and interceptors, I most likely would've lost at least half of my Empire. A Single Raven battleship of mine carried 3 Nano Rippers, 2 Warp Drive IIIs, and 3 Barriers, and it could easily take on any fleet of frigates and fighters and fight battle after battle after battle and defend multiple planets and starbases.
Speed is essential to any dynamic defense, and or, where you don't want to spend more than 20% of your income on your military... (Going for a tech victory

) Slow speed ships with just firepower and defense require you to buy more to protect planets because you have to station them at every planet. A small group of fast ships can quickly move from hotspot to hotspot. Military Starbases outfitted with Defense Assist (as opposed to attack), can make your dynamic defense even more impenetrable.
I caution against fast ships however, if you do NOT have a technological edge in firepower and at least an equivalent in defense tech. Without a technological edge, you need to devote more space to weaponry and defense, and so a more static defense broken down into sectors (ie, x amount of star systems that are close together form a sector), with a few fast ships to quickly reinforce is more in order.
Military Starbases and Planetary defenses are very useful and not very expensive if you're losing the technological battle and can't spare the economic points away from military to fund more research into better weapons and defense.