Gamespeed changes strategy significantly...
For one, not all variables scale the same...as an example, there is a 75% difference between the income levels on "slow" and "faster"...however, there is only a 40% difference between the build speed rates on "slow" and "faster"...because of this, when playing on faster speed you will find that units are more expendable and the double/triple frigate factory is a standard strategy for early game frontliners...
Many of the combat mechanics (ex: DPS) do not change with gamespeed...however, ship speeds (both phase jumping and normal travel) do change with game speed....you will find that on faster game speeds, surprise/flank attacks are much easier to deal with: the enemy fleet does the same DPS, but you can get your fleet their faster and build up defensive forces in a pinch much easier...
Specifically to what I said to Boshimi earlier, because DPS is the same regardless of game speed, slower game speeds make bombers/corvettes more important (because it takes longer to catch fleets or get across a gravity well)...
It's not just a slower game, it truly is a different game...things like when to get trade, when to get your 2nd cap, when to retreat your cap, what units to build, importance of scouting, etc. all depend on the game speed...
I personally like normal speed the best because it makes the game more relaxed but also more of a deep-thinking strategy game...slow speed though is just unbearable for me, mainly because phase stabilizers and bombers utterly dominate the game so much more than they do on the other game speeds....