Try these one at a time, checking after each.
See what happens in Troubleshoot mode (https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode/)
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Clear the browser cache:
- press Ctrl-Shift-Delete (Mac: Cmd-Shift-Delete)
- set 'Time range...' to 'Everything'
- untick all items except 'Temporary cached files and pages'
- clear, then restart FF
Turn off hardware acceleration:
- go to FF Menu > Settings and enter 'hardware' (no quotes) in the search box
- uncheck 'Use recommended performance settings' > uncheck 'Use hardware acceleration when available'
- restart FF
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Disable Efficiency mode: in about:config, change dom.ipc.processPriorityManager.backgroundUsesEcoQoS to false > restart FF
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Via the address bar, go to about:config > search for accessibility.force_disabled > change the value to 1 > click check mark > restart FF
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Disable VPN if you're using one
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Change proxy settings: FF menu > Settings > search for proxy > click Settings button > try the other settings
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Change DNS settings: FF menu > Settings > search for dns > try the other 'Enable DNS over HTTPS using' settings including another provider
Just above that at 'DNS over HTTPS', make an exception for the sluggish site(s)
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If using proxy and UBlock Origin, in uBO settings disable 'Uncloak canonical names'
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Disable security software http/ssl scanning if possible
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Check about:processes and about:memory
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If nothing helps, you could file a bug:
- via the address bar, go to about:crashes and submit all pending crash reports. Copy the ID of the most recent report.
- via the address bar, go to about:support > click 'Copy text to clipboard' > paste to a text editor and save it as a plain text file
- via the address bar, open about:memory in a new tab > click 'Measure and save'
- file the bug at https://bugzilla.mozilla.org. Choose the 'Report a new bug in a Mozilla product' > Firefox option.
- click the 'Attach New File' button to upload your saved files. Also mention your last crash ID.
