Hi and welcome to the forum
My 2 year old is good as new. You can search the forum for users that have had the phone for three years.
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Do you use an SD card? If so ensure it is formatted as portable. If not try and save as much of your data as possible and do a factory reset. Ensure any SD card in future is formatted as portable.
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Meanwhile start the phone in safe mode which will disable custom apps
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Clear the cache on the apps you use
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Read up on FP2 ( 6 years etc. to see a) how people are OK and how they got around the slowness.
On the battery. How do you charge it.
Allowing the battery to reach zero and charging to 100% often is very stressful although doing it now and then does help calibrate the battery but is not important to do this even on a bimonthly basis.
Have you checked the battery via the onboard system info.
Using the Phone keypad Dial *#*#66#*#* then Service tests > Test Single > Battery status
https://support.fairphone.com/hc/en-us/articles/360048843571