Nexus 7 (2012) Charging very very slow after flash

Hello guys,

Here's what happened with my N7 2012

-> Got lollypop update, it made my tablet almost useless
-> So i stopped using it for around 5 months - during this time tablet was in power off stage with around 50% battery
-> Yesterday i learned how to install factory image and successfully did so - now its running very smooth on KitKat 4.4.4 (KTU84P)
-> But it takes VERY long to charge this thing - it took 5-6 hours to get it to 50% (in power off stage)
-> I allowed it to drain completely and again tried recharging but its still very slow
-> Note that battery does not drain fast - once charged, it works perfectly and drains properly according to usage
-> Edit: I have checked my charging wire & adapter (Original Asus) - when used with other devices, it works properly.

I am prepared to buy new battery and replace it but what if my charging port corroded? - I inspected it but cant see any aging/corrosion with bare eyes - tried to clean it (dry) with a brush - no help.
Any suggestion / help will be highly appreciated.

Thanks
PS: if there is way to shut off annoying os update notification can anyone please share?