Mac slow, hot, laggy, and generally annoying to use

Did you install any anti-virus or so-called "cleaning" apps? Did your bank ask you to install an online banking security package from Trusteer (might be under the name "Rapport."

Are you running a client-side VPN? (the ones hyped in TV adverts all the time)

This Apple article shows how to use Activity Monitor (AM), an Apple utility pre-installed on your Mac, to see what is using up resources:

View information about Mac processes in Activity Monitor - Apple Support

The key to getting useful results is making sure AM's "View" option is set to "All Processes." Otherwise, the really heavy hitters can stay out of sight.

I am finding web pages that reman much too active even when in the background. Activity Monitor will show those as urls. Note in this screen shot that Amazon.com is using more CPU even in the background than the other pages and tabs that are also in the background:

Amazon can go much higher than, accelerating battery drain as well. The only tab more active was the Apple Discussions tab I was using at the time of the screenshot. That is why I close Amazon after I finish an order.

What all do you have running in background? How full is your disk? It needs to have approx 15-20% completely unused for the OS to run efficiently.