If you are looking for speed (and have some grasp of what you're doing) then your goal shuold not be to colonize but to decolonize the enemy's planets and detroys their ships; when your opponent send all of its ships out to colonize a planet, that's your cue to go to the planet it just left and decolonize it so they don't really gain anything.
+ to keep this comment alive please.
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