I have two Fairphone 2 devices here, one owned by me, one by my mother. She got hers in September 2017, I in January 2019. Mine has a few slightly upgraded pieces, like a better camera and battery, but is otherwise identical. I first installed Fairphone Open on mine and it ran smoothly. Then I installed it on my mother's phone, which was already slow with the old system.
When going through the initial setup, it was so slow that some screens stayed white for ~30 seconds and it didn't get any faster later.
So the usual performance troubleshooting steps like "stop background apps", "factory reset", etc. don't apply here. It's also not the operating system's fault, because I used the same on a nearly identical device. For that reason, I suspect a hardware problem, caused by higher than usual wear (she doesn't handle her devices too carefully). But how do I check that? Are there special tools that I can use to detect for example lower than usual battery voltage or check the CPU and RAM for faults or anything like that, which could help in narrowing down the cause or the performance issues?