The message you're seeing occurs when
(a) a webpage is poorly designed, overfull with content or simply on a slow server ; OR
(b) when something else on your computer is hogging its resources, meaning that Firefox can't do it's job properly ; OR
(c) when there's something wrong with Firefox itself, such as a malfunctioning extension.
Problem (a), above, should only occur with specific websites. If you're seeing the message across lots of websites we can probably exclude (a) as the cause.
To see if it's (c) that's behind things, try running Firefox with any extensions that you've got (such as an ad blocker) disabled. To do so, click on Help and then on Restart With Add-ons Disabled. (If you've not got 'Help' at the top of your screen, right-click up there and put a tick next to 'Menu Bar').
If the problem then disappears, try enabling your extensions one at a time (via Tools > Add-ons > Extensions) to try to identify which one is causing the problem.
You could also try resetting Firefox to its default state (via Help > Troubleshooting Information but, if you choose to do so, FIRST write down the names of any extensions you're using, so that you can reinstall them afterwards.
If that doesn't help, we've excluded (a) and (c), so we're left with (b). i.e. there's something on your computer which is slowing everything down. The most frequent culprit (by far) is a malfunctioning antivirus program. So which security program are you using, please?