I only encounter those symptoms on one Mac that is still on macOS 10 14 "Mojave" for compatibility reasons. It has an old version of Safari is not happy with some web pages, or vice-versa.
Our current Macs running Tahoe do fine but any Mac can be tripped up by useless add-ons. Most issues of "slow Macs" are due to unneeded aftermarket add-ons.
So please tell us if you are running any of these aftermarket products:
— anti-virus software
— so-called "cleaning" apps
— client-side VPNs ( the ones over-hyped in TV adverts; not a legit one issued by your employer)
— ad blockers (there are some reports that having more than one ad blocker can slow a browser)
— Safari extensions
The latter, extensions, obtained through the App store are usually OK and useful, but remember Apple does not test third-party software. If you have extensions, there is a procedure to look for suspects:
— Turn OFF all extensions in Safari Settings > Extensions tab.
(see: Use Safari extensions on your Mac - Apple Support
— Now test.
If the slows are gone, you had an extension issue and need to:
— Turn on one extensions only and then test.
— if speed is good, turn on another and test
Keep this up until the computer either slows or all extensions have been tested. If one extension makes the computer slow, you found a prime suspect.
Let us know. We may need to get you to post a system config report so we can look for less obvious causes of the "slows." There is a safe, secure procedure to post that we can give you.
