Performance issues with large data sets and pagination.

It is known that GraphQL is not the fastest API when you have many objects, see: https://github.com/graphql-python/graphene/issues/268

If you have many objects, you want to use pagination with DjangoConnectionField.

I have run some benchmarks for this:

1. There are 1000 items and i fetch 100 of them:

django v1.11.26
graphene v2.1.8
graphene-django v2.7.1
Use timeit statement: graphene.Schema(query=Query).execute('{Items(first: 100) {edges {node {id}}}}')
Run one timeit call... takes: 13.3 ms
timeit... use 5 * 75 loop...
max...: 10.84 ms
median: 10.66 ms
min...: 10.61 ms
cProfile stats for one request: 30148 function calls (28688 primitive calls) in 0.019 seconds

2. There are only 100 items and i didn't use a DjangoConnectionField:

django v1.11.26
graphene v2.1.8
graphene-django v2.7.1
Use timeit statement: graphene.Schema(query=Query).execute('{Items {id}}')
Run one timeit call... takes: 13.3 ms
timeit... use 5 * 106 loop...
max...: 7.54 ms
median: 7.48 ms
min...: 7.29 ms
cProfile stats for one request: 18556 function calls (17830 primitive calls) in 0.012 seconds

Graphene already makes a lot of calls anyway. But with these two variants there is also a very clear difference to be noticed.