Nighthawk R7400 Wifi extremely slow

You're at a loss because its a misunderstanding. 

2.4ghz is a slow network. My old networks in area's with 2.4ghz congestion, would only hit around 30-40mbps sustained. I might hit the occasional faster peak speed but not sustained. Thats the problem with 2.4ghz is that its slow, sensitive to interference, and there's limited  channels in which to choose from. Its speeds are even worse in apartments/condo's because of how close everyone lives. It also broadcasts through more materials than 5ghz which is again, worse for apartments as more networks interfere. 

I'd disable smart connect and every device you have that has 5ghz, should be on it. 

I have managed to hit faster 2.4ghz speeds but thats over an AX network in an area without congestion. But I still only hit maybe 100mbps. Using an RAX200 and RAXE500 and a intel AX210 wireless card. Again, in an area with much less congestion than an apartment. 

so hitting 30-60mbps in an apartment for 2.4ghz is actually a reasonable speed. (but sucks because their numbers say "up to 300mbps!!!")