TL;DR: DeJoy's changes appear to have made USPS slightly slower (0.1-0.5 days) in some places, but much less than the delta caused by a holiday like July 4th (see note buried at the very end).
Is my summary inaccurate? The article is very careful not to extend any political slant to its data and findings, please extend me the same courtesy. (I'm not even American.)
You say "slightly slower". The metrics behind the delays are much longer and your numbers only seem to include time in transit.
Second reason your summary is inaccurate: DeJoy's changes are rolled out in small pockets of the country thus far. The article seems to be indiscriminate from where those changes took place. The averages here are hence better than from those places where the changes have done the most damage.