I'm on iOS 10.3 right now (and had the betas).
Performance is bad, I think an iPhone 6 can perform better.
Tried erasing all content, no luck.
But I heard restoring it while in DFU mode will make it faster.
A few points.
First, A DFU restore may have different results than an erase all settings and content for one main reason. A DFU essentially reinstalls the OS completely. Erase all settings and content restores the device to factory state without reinstalling the OS. If you have a corrupted install of iOS on your device, an erase all settings and content will not fix the corrupted OS issue because it's not reinstalling.
Though it is rare for iOS to corrupt, it's no different than to any other piece of software, freak bugs cause even freakier glitches.
Second to that. If the OS is not corrupt, a simple erase all settings and content could potentially speed up the device because it is clearly out all the cached junk and removing any unnecessary files. This overtime bogs down the device which may cause performance issues.
Lastly, it is possible a hardware issue is at play. It could be a RAM or processor issue that is causing the device to perform poorly.
So you have already completed the erase all settings and content leaving DFU as your next step. If that fails contacting Apple to run a diagnostic may turn up something. I believe Apple charges to do it via phone or chat if you aren't under AC, but if you take it in, I believe they will do it for free.