Why is the midterm vote count taking so long in some US states?

In the House there are still more than 40 seats yet to be called, with at least a dozen of them highly competitive.

So what is it about the US electoral system that makes counting votes apparently so tortuously slow?

Responsibility for running fair and fast elections, like much of the way the country is governed, is devolved to each of the 50 states. How the count is done, and its speed, varies slightly between each state. (Election deniers have tried to imply that slow counts are somehow irregular or fraudulent. They are not.)

In this cycle, much of the heat is engulfing just three states: Arizona, Nevada and Georgia.

Several of the most consequential races are happening in the border state of Arizona. A US Senate contest between the Democratic incumbent Mark Kelly and Republican challenger Blake Masters could determine which party controls the Senate.

There are also consequential state races, including for governor and secretary of state, in which prominent election deniers endorsed by Donald Trump have a shot at winning. So far only 70% of the Arizona vote has been counted.

To understand why that is, you have to zoom in to Maricopa county, which covers the state capital, Phoenix. It contains 60% of all votes in Arizona and is the second largest voting jurisdiction in the nation.