Why do ISO's take so long to download?

That’s because they’re so fricking big! Try using Getright, if your download rates do not max out your download bandwidth. Its segmented downloading feature should help you to accelerate things a bit.

Yeah, but mine is only 544 megabytes! it has over 6 hours remaining. WTF?

Then I highly recommend Getright to you.

Does it really work as well as you say? Is the full version worth $20?

I used it free for years before I guiltily ponied up the cash for it. I couldn’t help paying; it’s the best download manager that I’ve ever used.

Sounds pretty good, I’ll test it out.

It sucks that you have to pay over and over, but it might be worth it. Do you think that trialpay thing is worth it?

You don’t have to use Trialpay. When I bought it, it was a one-time transaction. You can use the free version first. If you do buy it later, just pay the $20 and be done with it.

It depends on the size of the file, the speed of your internet connection, and the physical distance between you and the server you are downloading from (that is, if you are not using torrents). Before you shell out any money for a piece of software that only fulfills one single task, make sure you have all other factors covered.
As an alternative, you can also use the flashget download manager. That one is free and works great. Use the old 2.x version though, because version 3 sucks.

Yours probably isn’t the only request on a big file. The upload on a server has a limited capacity. This is why bittorrent was created.

ISOs aren’t special, they’re just ones and zeroes. They’re slow because there’s a bottleneck in the connection between you and the server you’re downloading from.
I downloaded 7 and XP from Microsoft’s website pretty quickly, as well as Ubuntu from one of its mirrors.