how to call some logic in my views after rendering template in django
You should use some queuing mechanism like worker and consumer to avoid this problem.
For example Celery. 
Steps to do for sending email:
1. Add email and info to the queue referred as task
2. Consume the queue. (It runs in the different process may be parallel too)
You can also use Channels newly added in Django family of apps.
This will provide you an asynchronous way to handle email/any other deferred task.
I am using form.py and the user is typing some Email-id, let us say I want to send an email to that particular email and write all that email into google sheet using gspread, I am able to do this in my views.py, but the problem is it's taking a lot of time to write which slow down the rendering process.
Is there any other way I can use my logic after rendering the template.