system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that in certain areas my mobile phones Internet will be extremely slow, despite having full bars, and sometimes the speed will be fine even with one bar?

This bugs me a lot.
gpt
There's three main reasons, and unfortunately none of them will be fixed any time soon.

The first is that there's no universal standard for what bars of signal actually mean. Every manufacturer has a slightly different way of working out what it shows on your screen, and some will even vary it from phone to phone. They're not trying to fib you into thinking you have great signal when you don't (usually), they're just trying their best to come up with a solution that actually makes sense. Which is hard, because there is no single number that can express how good your reception is at any given time. So full bars might not actually be the best signal, and one bar might not necessarily be the worst.

Secondly, the world wide web runs on a system called TCP/IP. It basically tells two devices how to send a message between themselves in such a way that they both understand it. Now, TCP/IP was designed before wireless data transmission of that kind was really a thing, so it was designed on the basis that you'd have a solid, reliable connection like a network cable between the two devices. Now wireless is less reliable than that. Not unusable, but a bit shaky. There's all kinds of things that can interfere with the signal, and because TCP/IP was never really built to work around that, it just tries it's best. It does pretty darned well, too, but it's just not as good as a wired connection. A lot of very smart people are trying to make it work better, but that could be a while off yet.

Lastly is a thing called "contention". In the context of mobile internet, that means "how many people are trying to use the same connection". Because your internet connection is only wireless until it gets to your carrier's transmitter (the thing that throws out a mobile signal), and then after that it goes along a broadband connection not unlike the one you'd have at home. But it might end up serving dozens of different people, depending on how many are in the area. So even if you're sat on top of the transmitter with a perfect connection to it, there might be lots of people using that same internet connection and slowing it down. Your mobile phone only shows bars to indicate how well it's connected to the transmitter, not the internet beyond, so you have no way of knowing how bad the contention is. 

The same problem does exist to a degree with wired broadband - some providers might give you 50:1 contention, meaning fifty people using one connection - but a busy street might have a hundred people walking down it with phone downloading something as they walk, plus another few hundred in the surrounding houses. The line gets busy. At a concert in the sticks you might suddenly have five thousand people on one transmitter. This used to be a really big deal, because a lot of mobile networks never really thought this mobile internet thing would be as big a deal as it's turned out to be, so their infrastructure was pretty shoddy. They're working on it, but not all networks are as good at getting it right.