After flashing the phone from android 6 (pre 20160822) to 7 (20170708), the phone became stupidly slow, opening facebook or viber takes about 20-30 seconds, other apps got slow as well. Can this be solved by downgrading to earlier ROM, or should I just go back to 6.0?

4G does not work either, never did actualy, I was hoping that an update will solve this matter, but it did not

I will report this. You may try out flashing other versions. Please also try a wipe/factory reset after flashing.