And what do you consider crappy speeds? Great you have a dual band N router, for all we know you have a B card in your devices?
Also your channels - you mention channel 7 for Extension??
What is that? You do understand that only 3 channels that doubt overlap are 1, 6 and 11 here i the states. So 7 and 11 are not far enough apart to prevent interference.
You say only thing you have done on router is change the password?? What mode are you running in? N only, BNG, are you on 2.4 or 5 band? Are you using 20 or 40 mhz for channel width? With a N750?? Hmmm could that even be set for 5ghz 80mhz channels? So you have the WNDR4000 netgear? If so doesn't look like the manual lets you set channel width -- their interfaces are so limited it lets you pick your top speed
up to 145, up to 450, etc. Without actual details.
There are a bajillion things that could be going on - you have given us nothing to go on other than you believe its not as fast as your wire.. Well no ****
Like telling your mechanic hey car wont start - whats the problem?