It's the amount of units you destroy and buildings you capture.. so if you think you are very good speedwise - go capture more before completing... also makes easier missions harder, considering you need to put some turns in making motorcycles for more city captures..
Speed is usually hardest for me because I sometimes take too much turns capturing to secure a good strength...
But it all depends on ofc.. I wish someone would post the turns needed for 100% of all missions, would make things easier... getting around 90% speed seems fine as long as other 2 are over 90% as well for 3 stars...