This could be a million things. What steps have you taken to diagnose the issue and what were those results?
What speed is your circuit, how many users do you have, and does the uplink utilisation show you have plenty of capacity?
What does speedtest.net report?
Web browsing is broken into several steps, resolving the initial DNS name to an IP address, and then downloading the page, resolving all the remaining DNS entries and then download all of those components.
Do you get a fast response using nslookup?