Why is counting votes so slow in some US states? Are they slow counters or do the have a problem with larger numbers?

People who think elections can be hacked or stolen are insane. You Trumpets need a reality check. Biden is the president. Get over it. 

Trump's postmaster general is making the mail-in ballots arrive real slow.

Having many extra falsified ballots that need to be added to the mix discreetly always slows everything down.

Also, like Stalin said, "It's  not who votes that matters, it's who counts the votes that matters."

It would also help if the GOP cared enough to have observers from their side BE There from the beginning

The process of checking and counting mail in ballots is slow, and more intricate than with day of votes.

That's a good thing.

Because of the pandemic, there are both a LOT more such votes, as well as due to Fat Donnie telling his voters to NOT vote by mail (Even though he DID), that means that the day of votes and the mail in votes are going in different directions, while in prior elections, there were so few such votes, and they generally went about the same way that day of votes did, that means that you cannot project how they will go this time until they get counted, and the remaining total to be counted drops to under the margin between the two candidates.

As some of the responses indicated, each state has a process for counting ballots. Some allow immediate processing of ballots once they are received (thru the mail or in-person early voting).  Some, by law, require that the ballots be counted AFTER the polls close.  So, they have millions of ballots to process and that takes time.

Many states, as in mine (Ohio) have paper ballots that need scanning.  This means opening the outer envelope (checking signatures and matching barcodes), and then, opening an interior envelope with even more information that needs checking- then, taking the ballot out, flattening it so it can be scanned.

So, it's not as easy as some think. Not all counties in the states have electronic voting.  It's a long process and the Boards of Elections in these states want the process done fairly and accurately.

Justice Alito just said "All votes received after 8:00 PM on November 3rd in Pennsylvania are to be held for review.  If there is a problem and I'm sure there is, someone must answer for this voting fraud in America.

Notice: those are DEMOCRAT states. 

They are trying their best to invent votes and force Trump out of office. 

In Pennsylvania, there was no early voting, only in-person or absentee. Something like 90% of Trump's supporters went in person, as he instructed, and something like 90% of Biden's supporters used the mail-in, being prudent about Covid. (Yes, we're still in the middle of the pandemic, it was not a mirage like the Trump lead was.) 

Think there are still another hundred thousand or so to be processed in PA. It's a slow process, and it would go quicker without Trump's lawyers trying to force "Cease & Desist" every few hours...but they're not interested in an accurate count, they only want the ones who voted in-person to count. 

Trump's been trying to pull the same thing in AZ & NV and that's slowing it down out there too. 

Wish he'd go on and admit the inevitable. A president who's only supported by 48% of the people isn't likely to get more than 48% of the vote.

I suspect they are recounting because it's so close and before they publicly release the results.

The laws differ between states.

Some allowed the counting as the votes came in. In others, they had to be piled up and saved, and counting was allowed to begin only AFTER the physical polls were closed.