High download speeds, slow browsing speeds
It could be that there's a DNS problem somewhere. Web browsing tends to involve a lot of small connections, with several DNS lookups just for one page, whereas large file downloads only need to make one lookup. So if there's an issue with your ISP's DNS servers, it could be slowing down your web browsing without affecting your large transfer speeds.

Try switching your DNS to use the Google Public DNS servers. Open up the Network pane in System Preferences, select your wifi and click Advanced…. Then go to the DNS tab, and enter in 8.8.8.8 and 8.8.4.4, as shown in the screencap:

Make sure those entries are above anything else in there.

It may be another issue, but this is a good starting point.