I actually really liked A12’s look but, performance for me is everything, and all the ports the A12 developers had to do, killed performance. Nobody knows why. Also I too had Wi Fi signal issue on A12 ROMs. It was very bad and slow.Yeah I think you're perhaps right. It's probable the Nokia 8 doesn't even officially support Quick Charge despite having the capabilities to do so. It's been so long since I used the stock ROM that I don't remember. It doesn't state anything about QC on the phone's box.
I tried switching off the phone completely and retesting the charging speed in offline mode (without booting Android) which gives the same speed of around 1.4A. So I guess that confirms it is nothing to do with the kernel.
I agree it's frustrating that the clock speed is limited in the stock kernel. I don't understand why they do that. The reason I switched back to stock ROM is mainly due to the WiFi signal. In the custom ROMs the signal is terrible and constantly disconnects from the router. I don't have this issue in stock ROM. I also hate the Material You redesign in A12 and refuse to upgrade to that version.
About the clockspeed limit, Nokia isn’t the only one who did that, which is even more infuriating. Nokia 8 Sirocco seems to have no such limit