I am by no means a networking guru, but I do work in the IT industry so I know some things more than the average person.
One good example is this – If you connect to your wireless router at 300mbps, it does NOT mean you will get 300 mbps speed. It means you can get “up to” 300 mbps, best case. Real life situations often cut this speed by 50% or more. Why? Because of all the electromagnetic signals being generated by everything around you – your phone, your TV (worse still if CRT TV), fridge, microwave etc.
As these signals float around your house, your router and your device (phone, tablet, laptop etc) need to overcome these interference by doing error-correction. This may take micro-seconds or nano-seconds per packet, but a continuous stream of error-correction will slow your wireless speed down. Just imagine this – the speed limit on the highway is 100km/h. That is your connect speed, the highway. Your car can travel up to 100km/h at most ideal situations, but if there’s a car in front of you going at a slower speed (interference), you need to slow down, or change lanes. “Error-correction” is changing lanes – you spend some time doing it, a couple of seconds, but it does slow you down. Imagine now the highway is full of slow cars and you need to keep changing lanes – that is why your actual speed and the connect speed will never be the same.
Secondly – no matter how fast your router advertises your wireless connect speed is, you can NEVER go faster than the speed plan you purchase from your ISP. It does not mean if you connect at 300 mbps wirelessly to your router, you will get 300 mbps speed when your internet plan from your ISP is only 100 mbps.
Third – if your router is only able to let you connect at 300 mbps and no higher, buying a better USB wireless adaptor (eg an AC1200 one) does not mean you get faster speeds. It will still connect only at N-speeds, ie the best your router can offer.
Fourth – wireless connect speeds, for most intent and purposes, are irrelevant. Imagine it as an invisible cable hooking your phone/tablet/laptop to your router, that’s it. The speed you connect at, wirelessly, is how big the highway is between your device and your router, NOT the internet. This is significant only when you need to copy files to other devices at home, where a higher connect speed means you copy the files quicker. See point 2.
Fifth – in some cases, uncommon but not rare, your older laptop/PC/phone etc will actually kill your wireless speed. The CPU may not be fast enough to catch up to the speed of the wireless, so your actual speed will drop.
Sixth – the best advice is still “connect via cable whenever possible”.