Slow internet solved

I have 2 computers sitting 3 feet apart, on wifi one was getting 600 Mbs and the other getting 3 to 7 Mbs same linux operating system, i searched google for all the fixes, nothing really helped much. I checked the specs on the wifi card and it  should be as fast as the other computer. Well i had a older compatable wifi card i got out of a older computer i had, i installed it in the slow computer and now i get roughly the same 600 Mbs. It seems the other wifi card was faulty, it worked but not very well, just goes to show its not always Comcasts fault.