Lenovo — Phone is operating very slowly

Address:

Hyderabad, Andhra Pradesh, 500050

I have purchased a A6000 mobile in Feb 2015, initially it is working good but 2 months back the Android is upgraded from Kitkat to Lollipop version and it start troubling too much.
It is operating very slowly and some times not even displaying incoming call.
If mobile is connected to internet or WIFI then phone will be hanged for about 15 to 20 minutes.

I contacted the customer care person, he said again Kitkat version need to installed and hand over the phone for a couple of days.

I purchased the phone with lot of trust on Lenovo but now the support from lenovo team is very bad (not even expected such a bad service)