I confirm, I have the same problem.

Make sure you don't have any other programs running in the background except for steam.

You mean using a gaming computer for cook food :

Well, having a 7 year old computer, i have no problem with Rebellion and slowndown... all setting on hightest, filter and shadow included, custom ultra huge random map ( 10 star, 4 pirates bases by star, 10 player, over 350 planet )... 10 hours play, everybody maxed fleet and planet, insane novalith bombardment, etc ... and the game is only using 1.1gb ram and not slowdown... only a little frooze ( 1 second or so ) when i zoom in and out very fast, multiple time...

The only problem I have is immense lag when a large fleet jump to another star at the same time. (when the phase jump loads)

Is not your 7-old computer dual-CPU system?   

@ Teun-A-Roonius> with all the respect, great notebook video-card is an oxymoron.

number of gigabytes is fairly irrelevant. the number of shader computing units is what makes the GPU strong, and notebook gpus are generally pretty lackluster in this regard (because lot of shaders = lot of Watts needed). There are however few high-end mobile GPUs, which are spec-wise closer to their desktop brethren, maybe you have one in your macbook.