Hey wajid225,
We understand its been 2 days since updating to iOS 12.4.8 on your iPhone 6 Plus and it's freezing up frequently.
By reset, what exact steps did you take? Did you restart your iPhone, reset it's settings, or restore it as a new iPhone?
Also, so that we can rule out any issues with your iPhone's battery as the source of the performance issues you're facing, can you let us know what the max capacity is currently at for your iPhone's battery? You'll find this by going to Settings > Battery > Battery Health.
If you're close to 80% then your iPhone battery is near consumption, and if it's already lower than this, you'll need to replace your battery to restore maximum performance.
For more info on how your battery can affect performance, check out: iPhone Battery and Performance
Take care!