Just an idea: As I described here, I suffered a similar delay but especially a delay of every alarm tone. I found the stock messenger app blocking the memory by gigabytes of its cache. Maybe you’ll find a similar memory blocker, maybe even the messenger. So simply emptying the cache solved the problem.
Nevertheless I can confirm, that my FP2 works still quite well (still Android 6), (it’s not the fastest device on the planet, but fast enough for me). My problems were hardware related (battery, display), and I had to learn, that a defective display is able to reset the phone, because it managed to type in the wrong unlock PIN five times, confirm the warning about the only three trials left and type it wrong again three times. So I switched it on and was asked: “Please choose a language…”
So I would never never never use any device like that for important information without a backup. MyPhoneExplorer is pretty good for that, but it needs patience if the phone is quite full. For banking I tried to get two different identification methods for every account, so that I can unlock one by the other. And the most two factor authentications only based on the phone number (just put the SIM in another phone), or based on apps, which only need a passphrase after reinstallation.
Yes, I don’t like to build up my phone from scratch, but the above mentioned crash forced to do it, and I only lost some minor important data.