In 20,000 range, if you want serious performance and no lags, Window phones are best. I tested multiple lumias personally in extreme conditions, compared them to 1GB ram Androids in heavy use and found out that WP don't lag, where as 1GB ram Androids lag in heavy use, multitasking etc.
Check Lumia 540 dual sim, ~Rs. 16500. Windows 10 update coming very soon for it.
http://www.microsoft.com/en-pk/mobile/phone/lumia540-dual-sim/
Much faster and better than Grand Prime, except for camera. Lumia 540 IPS display is 294 ppi and you will find Grand Prime 220 ppi non IPS display blurry in front of it.
For an experiment, i bought the most basic Lumia 430 and it even beats my Core Prime 4G in multitasking and heavy use [emoji1]. I also plan to go for Lumia 540.
Shame on Samsungs!.
Have a look at windows 10 mobile UI.
https://www.pakwheels.com/forums/showthread.php?p=4874835