For our hopium-enjoyers, there is at least one plausible explanation for the delay. Joe's a little under the weather and sundowning, so they spike him full of even more of the good drugs, delay a bit for them to take effect, but they don't get him feeling right. Time to give him the ol' nasal swab; rapid antigen takes 15 minutes, they get that back, and, "shit, better run the table top PCR/LAMP/whatever magic the president has access to". Burn another 15-30 minutes checking and double checking results. Run the "oh shit he actually has COVID" news by the campaign staff/handlers to figure out whether/how to break the news, add a bit more delay. So, possible that that's happening.

That all said, I have a hard time imagining they're not running a desktop assay for a shitload of stuff multiple times a day given old as hell president; not doing that would be a startling show of incompetence. On the third hand, should I really assume anyone allowing Biden to run for president is competent?