Why is my internet speed slower than what I am paying for?

[SAI]

Long story, short:

I'm paying for 25 Mb/s, but on speedtest.net, I'm only getting 5 Mb/s.
I know it isn't fully accurate and that it's suppose to be 25 Mb/s max, but is it suppose to be this much of a difference? 5 Mb/s is suppose to be the speed package tier below the one I'm paying for.

I already contacted them and they guided me through some steps but that didn't fix it.

I also know it isn't the computer, because it's the same speed for both computers I have. I've reset the modem, cleared cache and stuff, but no success.

Any ideas on what I should do?
Additional Details: Both computers are fairly new. One was bought in 2008, and the other 2011, and both their speed use to be fine, but suddenly dropped down to 5 Mb/s max.