You need to isolate where the slowness is coming from. Are you wireless or wired? If wireless, test it with an ethernet cable. Also try with another computer if you have one. Test the connection going directly to the modem. Try rebooting the router and/or modem.
You should have a check that the cause is not a virus on your computer.
Or any sort of malware.
A few weeks ago i had to go and look at some ones computer because it was doing exactly the same thing.
It turned out that a Bitcoin mining program was the cause as a background task.
Using about 70% of his processor time to remote mine for bitcoins.
Im careful, but some how it managed to get on my machine also, Everything went to a crawl including loading web pages. But knew by then the cause.
There it was mining away using 70% cpu utilization on my system.
Please click on this link and download the free version.
Install it on your computer, and run the program.
and click scan.