Trump has long been a critic of the postal service, but his latest charges come as he trails Biden in most polls.
Protesters on Saturday gathered at DeJoy's Washington home, blaring horns and banging pots.
But USPS spokesman David Partenheimer attributed changes at the agency to its poor financial state.
"We are not slowing down election mail or any other mail," he told AFP.
"The Postal Service is in a financially unsustainable position, stemming from substantial declines in mail volume and a broken business model."
But Congress remains deadlocked over a new stimulus bill to follow the $2.2 trillion package passed in March.
The president said candidly in April that mail-in voting "doesn't work out well for Republicans". He has repeatedly described such ballots as prone to fraud.
But a study this year by New York University's Brennan Center for Justice found that "it is... more likely for an American to be struck by lightning than to commit mail voting fraud."