Building on the answer from @chris

The OP stated:
"...and knowing it is made of just one key/value pair..."

In this case scaling of execution time is irrelevant.

However next(iter(d)) is still faster even with a single key:value pair.

>>> import timeit
>>> setup="d = {'key': 'value'}"
>>> timeit.timeit(stmt='list(d.keys())[0]', setup=setup)
0.14511330000823364
>>> timeit.timeit(stmt='next(iter(d))', setup=setup)
0.07170239998959005