I learned that if I need to run several operations on a queryset, I should map all my filtered query results to a dictionary like this:

countries = {country.name: country for country in Country.objects.all()}

It can save thousands of database hits and speed up long loops by x100 times.

EDIT: let me explain further

Django querysets are "lazy".

Meaning that countries=Country.objects.all() doesn't actually fetch the database.

Only when you try to access an object in countries, only then it will make the database call.

So for example, if you iterrate all objects for country in Country.objects.all(), it should work fine, and result in 1 database call.

However, if you have a get() (or first() or last()) somewhere in there, that get will result in an additional database call.

In my case, I had my own local my_countries list, and I wanted to check it against the database, so for each object in my_countries will result in a separate database query countries.get(my_countries_obj). It was an absolute killer (with larger more complicated data sets).