Your browser can only display stuff as fast as it's getting it, and the cache would only effect images the second time you downloaded them.
You've not really given a lot to go on (and not answered my question) so it's a bit hard to say anything, but there are hundreds of reasons why downloads could have gone slow, ranging from changes at the exchange or in the local area (I've seen somebodies speed plummet as a local, new, estate got populated!) to something infecting your computer and using the bandwidth.
Downloading and running Ccleaner will clear out all your temp files and can often help
http://download.cnet.com/ccleaner/
(note. default settings will clear all your saved login information for websites, so make sure you know them)
It's also always a good plan to run a full scan with your anti virus and malwarebytes to double check you've not got any nasties, though you can also check by closing everything on your computer and looking at your router lights, the occasional flash is fine, but if the lights are going mad when the computer should be doing nothing it's worth checking.
Other than that, check your speed over a period at different times so you can get an idea of what sort of speed you are getting.