There could be any of a number of things going on.
Are you on Wi-Fi or are you hardwire connected to a router or to a modem?
How is your Internet speed loading other sites? We use Wi-Fi for just about everything now, and there are times that I have to go and reboot the whole mess (cable modem and Wi-Fi Access Point/Router). I just cycle off the power switch to all of the networking devices, and once they all restart and reconnect, everything usually works much better.
If that is not your problem, then those making jokes about porn could be on to something--not necessarily that you have been launching porn sites, but that you may have viruses and malware slowing down your computer.
Have you restarted the computer and it's still doing this?
If you don't know that you have good virus protection installed and updated, go to Malwarebytes Anti-malware, download their scanning software, install it, update it and let it scan your computer:
https://www.malwarebytes.com/mwb-download/
Let it clean whatever it finds.
Select the Windows or Mac version--whatever you're running.
Other possible causes--on an old computer, your hard drive may be full! That will slow down any computer to a crawl.
I'm kind of shooting blind here and I kind of need to know what you've already checked.