The difference here is that you're working under the assumption that Network Latency doesnt effect a game. But thats just wrong. It's real. So real that the DoD has been working on ways to reduce it using a method called "Dead Reckoning". You simply cannot say that one map plays differently on a different speed because that simply is NOT the case. If it's easy on one speed, its just as easy on any other speed. The only difference is the network latency between each speed is different. The biggest difference in latency compared to all other speeds is Fastest, it has the WORST latency of all of the speeds. So no matter how much skill you think X map takes, the lower the speed, the more of your own skill you will be able to exert on that map. So if a map plays "best" on Fastest, it's because that map is so easy that you have to introduce impossibilities into it by increasing the network latency. And how do you do that? Simple, increase the game's speed because that effects the game's network latency as well. You're working under an illusion if you think game speed has anything to do with how hard/easy a map is.
So any map that "plays better" on fastest is also "an easy map", so easy that you have to make it impossible for your units to respond correctly, otherwise, it becomes to easy. So this is why you are experiencing "well its to easy on EF". It's not the map nor the speed. It's the network latency that is increasing your ability to show off your skill that is making it "to easy".