In my experience, the party at fault in delayed delivery has been more often USPS than not. The item lies there at the airport, past the customs, and USPS does not pick it up for 2 weeks or more. When items have been coming in from UK or Germany, they got picked much faster. These delays started as the anti-Chinese rhetoric was ramped up during the previous presidency, but they did not go away with the current.
There have been pauses for a while. I don't think it is due to any one administration or another.
Packages coming from China are shipped dirt cheap. I don't know the exact charges, and how it is distributed between China and the USPS. However, they appear to come in at about half the cost it would be to ship a package within a single city in the USA. Potentially at a loss for the USPS.
It may be that the USPS puts lower priority on Chinese shipments than other shipments.
One might note. Some Chinese vendors on E-Bay and Amazon may have local USA warehouses. Not necessarily without delays, but they may be quicker than coming from China.