After writing that last post, I got a great idea.
What if my wife's phone doesn't have the GPS problem my phone has/had? If her phone functions properly, I can simply switch the phones because she doesn't drive the car yet. We both have the same phone so she won't miss a thing.
Ideally, I should have tested her phone a few days to see if it was working properly, but I was impatient. I wanted to switch first and then test. Then at least I would be able to use Marshmallow again. The feeling of being able to use the latest OS makes me happy, even if it is slow.
I factory reset both phones and flashed the latest version of Marshmallow available on Google to both phones. I swapped the SIM cards and the phone cases.
I restored the applications from the corresponding latest backups to both phones. That meant installing a lot of applications than what Marshmallow was used to running on my phone, because my final backup was from KitKat and I had installed a lot of app on it because it never slowed down like its successors.
This immediately brought back the sluggishness I was so used to on my phone. I used the phone the whole day yesterday and it eventually became somewhat usable. Either the indexes/caches we constructed to make things faster or my body got used to the sluggishness. Or it can be both.
What about the GPS issue? Is it better on this phone?
Yesterday I had to drive for about 30 minutes (I had to visit a house in Mulgrave to check some furniture which was advertised on Gumtree) and I found the location to update without a hiccup on Google Maps. Does this mean the problem is fixed? No, because it is definitely too early to draw any conclusions just yet but the initial results are at least positive. I think it needs at least a whole week of testing but I don't drive much, so a week of good performance is still not a good indicator. We shall see. I'm sure you'll hear more about this topic soon.