Two days after the US midterm elections, a sense of deja vu is descending over the country. In a replay of the excruciating events in 2020, when Joe Biden’s presidential victory was declared four days after the polls closed, Americans are yet again asking themselves why they have to wait so long for election results.

Later in the week, it remained elusive which main party will control both chambers of Congress. In the Senate, Republicans hold 49 seats and the Democrats 48, with two states – Arizona and Nevada – not yet called, and Georgia headed to a runoff.

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?

Where are counts still happening, and why?

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.)

The big picture here is that counts are taking extra time in races that are very close. News networks are hesitant to project winners because the margins between candidates are narrow and there are many ballots left to count – and so the need for patience may be justified.

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

What’s going on in Arizona?

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.

The number of people who vote early has increased dramatically since the pandemic. This year Maricopa county also saw a surge in the number of early ballots that were dropped off on election day – they are known as “late earlies” – rising to 290,000, the largest number in the state’s history and 100,000 more than in 2020.

Each early ballot has to be verified to check that the voter’s signature matches the signature in the voter rolls, and after that is done it is sent to a bipartisan panel for approval and processing. That all takes time, as we are witnessing.

Many people have drawn a comparison of Arizona’s vote count with that of Florida, which called its results within hours of polls closing on Tuesday. That state’s system allows election officials to begin counting mail-in ballots as soon as they are received; mail-in ballots have to be requested and must be received by an election supervisor no later than 7pm on election day.

But the main reason why Ron DeSantis won his re-election race so quickly on Tuesday was because it was a blowout, with the incumbent Republican governor garnering 59% of the vote while his challenger, Charlie Crist, received only 40%.

Had the candidates we are watching in Arizona or elsewhere had such a convincing lead, we would probably not still be waiting for their races to be called. Nonetheless, there are questions that Arizona is going to have to face in future elections.

Stephen Richer, who is the recorder of Maricopa county, said that after the dust settles “we will likely want to have a policy conversation about which we value more: convenience of dropping off early ballots on election day or higher percentage of returns with 24 hours of election night”.

What about Nevada?

Nevada is going a bit faster than Arizona, with 83% of the votes counted, but this year the count could last through Sunday. But like in Phoenix, there are still large numbers of ballots yet to be processed in the big urban areas of Las Vegas and Reno.

The state runs its elections largely through mail-in ballots, and that in itself bakes in time. For a mail-in ballot to be counted it has to be postmarked by election day, but the state now allows until four days after election day – 12 November – for the physical envelope to arrive.

There is a debate to be had about the merits of such a system. Many election officials stress that it is more important to have a system that is convenient, accurate and accessible than one that is fast.

The count in Nevada also has a lot riding on it. That includes a very close race between the sitting US senator Catherine Cortez Masto and Republican challenger Adam Laxalt; three tight contests for US House seats; and a battle involving one of the most visceral election deniers, Jim Marchant, who is running for the job of top election official.

And Georgia?

Georgia has completed its returns for its critical US Senate race, with the Democratic incumbent Raphael Warnock squeaking ahead of the Trump-endorsed former football star Herschel Walker. But this state runs a system whereby if neither candidate marshals more than 50% of the vote – which neither did – there has to be a runoff election.

That feels like groundhog day too – we had to wait until the January after the 2020 election for two Georgia runoff contests to be called before we knew that the Democrats would control the Senate. At least Georgia has speeded up the process: the new voting law SB 202 has significantly shortened the period for this runoff, which will take place on 6 December.