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.
In addition to what already was mentioned: How is your content filtering on the MX configured? If it is "Full list" try "Top sites only" and look if the problem stays the same. I had some installation that had the same symptoms with "Full list".
It is brand new network i made it with L3 switches.
before was like All L2 switches going to DC via 1 Gb fibre and connected to another L2 and then via That L2 in DC connected to same MX, but we have no issue at that time.
then we changed it to 10 Gb connectin between 2x L3 switches in dc and campus and problem comes up, as i mentioned for now after change the DNS and also some more work on Client PC seems ok but still some users having issue somtimes.
first page loading so slow or sometimes get error loading page.
internet bandwidth is about 200Mb/s- but we are using this bandwidth for MX's auto VPN of other branches come to this MX from another site as well.