You can try to modify multi-process settings to see if this has effect.

• set dom.ipc.processCount to 1 if it is currently set to a higher value (4)
• disable multi-process windows in Firefox

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

• http://www.ghacks.net/2016/07/22/multi-process-firefox/

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for the quick reply. I'm testing out the autostart tweak to see if it works. The other one was already set at 1. Will report back in a day or so. It seems the autostart.2 was set to true

Update: fix did not work on the desktop(autostart). still waiting to see if there is a slowdown on the laptop.

After about 6hrs there was a sudden and extensive slowdown/lag while in safe mode. Exiting out of the program and restarting works to fix the issue but this is quite troublesome.

Now have moved to an even newer late 2015 imac. Same issue persists. Very very irritating.

This is happening on now all 3 of my machines.