Looking at Google -> US elections, I see that in many US states the percentage of reporting is >95%. This leads me to think that the last few percentages of votes in some states take much longer to count than the first ~95% of votes. Why?
Federico said in a comment
lots of small counties and not so many huge counties
That's part of it. Another part is that mailed ballots take longer to process. They must be opened and then read, and the vote then recorded. Where both in-person ballots and mail-in ballots are counted by a scanner, the in-person ballots are counted as they are cast, and the total is available on each machine instantly when the polls close. All that remains is to transfer the count from the machine to whatever central tallying process is underway. Mail-in ballots have to be fed into the scanner.
Azor Ahai -- he him raises another interesting point in the comments, which is that problematic ballots are counted last. These will generally be more time consuming, so even if they are counted in parallel they won't be completed until later on, and they will in general represent a small proportion of the total count.
To (hopefully) clarify a comment for people who might not be aware of this: when you see reporting percentages, those are frequently not percentages of votes tallied; rather, they're percentages of voting precincts reporting. Precincts aren't uniformly sized, and precincts with larger groups of voters can take quite a bit longer to count than e.g. the Dixville Notch vote in New Hampshire, where the votes of all 5-10 eligible voters can be tallied in a matter of minutes after the polls open at midnight.
Because another Q was deemed to be a duplicate of this Q, I want to offer an answer to the question (from the closed Q): That "new" Q asks:
Why is the pace of vote counting so fast on the first day? Why does it slow down afterwards?
Consider the County where I live: On election day there were 212 precincts, each precinct has one or more voting machines. So that on election day there are actually 212+ machines that are counting in person votes simultanously.
Contrast that with the 3 counting machines (at the County Board) that are scanning all the mailed ballots. Even allowing that the precinct machines do not process at the same speed as the more expensive high speed (at the county central), having more than 60 times as many machines running simultaneously, more ballots can be counted per hour.
Notice that I have not addressed the pre-canvass requirements. In my county there are (about) 450-500 people "checking-in and validating" each in-person voter at the 212 precincts prior to issuing a ballot. Contrast that with the (about) 25 people who are "checking-in and validating" the mailed in ballots prior to those ballots being counted.