Minimizing render times of shared Django forms

A common situation with Django sites is the need to render a given form across all pages, such as a login-form that is embedded in the header. There is a recipe I came upon, probably from stackoverflow, that has some derivation of the following pattern:

# as a context_processor
from .forms import SomeLoginForm

def loginFormProcessor(request):
    ctx = {}
    if not request.user.is_authenticated():
        ctx['login_form'] = SomeLoginForm
    return ctx

# your template
{% if not request.user.is_authenticated %}
    {% crispy login_form %}
{% endif %}

I was using this pattern for a rather complicated form without thinking about the overhead incurred. However, when new-relic revealed this was taking ~600 ms per render, I knew it had to be fixed.

 The simplest solution is template caching, making our template look like so:

# your template
{% load cache %}
{% if not request.user.is_authenticated %}
  {% cache 99999 login_form_cache %}
    {% crispy login_form %}
  {% endcache %}
{% endif %}

The problem with this is we still incur the overhead in our context processor. We can avoid this by doing all our work within the cache tag. First, we need to move the logic of generating the form out of the context processor and into a template_tag.

# our template_tag.py file
@register.assignment_tag
def get_login_forms():
    from ..forms import StepOne, StepTwo, StepThree
    ctx = {}
    ctx['first'] = StepOne
    ctx['second'] = StepTwo
    ctx['third'] = StepThree
    return Context(ctx)

Now, we need to integrate this tag into our text, so our final template looks like the following (this is also more related to my particular example where I have a multi-stepped form):

# our template file
{% load cache our_tags %}
{% if not request.user.is_authenticated %}
  {% cache 99999 login_form_cache %}
    {% get_login_forms as modal_login_forms %}
    {% crispy modal_login_forms.first %}
    {% crispy modal_login_forms.second %}
    {% crispy modal_login_forms.third %}
  {% endcache %}
{% endif %}

This alone made the server response time come from ~2-3  seconds down to 0.69 seconds.  Not too shabby.

Note: This code should run but I didn't test it as it isn't exactly my code copy & pasted, but an example.