Phone extremely slow, even after factory reset

I have a Samsung Galaxy J5 2016. It's gotten extremely slow and buggy (>10 crashes a day, waiting more than 30 seconds to open default apps, ...). I've done a factory reset, with removal of all data, including SD card. And... It's just the same. I'm already looking around for a new phone, but I'm wondering what can cause this? When I google why phones get slower over time, they point to more and heavier apps. But this doesn't explain my case. What can cause this behavior, even after a reset?