Even the tracked Priority Mail packages have delays. The system promises 2-3 day delivery (and that's what customers are told during checkout, no way to change it) but it's often longer than that, particularly for larger/odd-shaped packages such as the long Priority Mail tubes.
In New England, a lot of problems can be traced to COVID outbreaks among USPS staff in processing centers. It was especially bad starting in November and continuing through February.
At one point the Post Office stopped scanning parcels (for us, in mid-December) and huge backlogs built up across the system. We heard from our USPS business rep in Boston that 200 workers were out sick at the giant processing facility in Nashua NH, leading to trucks arriving at the parking lot that couldn't be unloaded. They switched to processing Priority Mail at the facility in South Boston and everything else in Stamford CT, but they weren't able to work through the Nashua backlog until January. One Priority Mail parcel we shipped on Nov 30 that was supposed to be "3-day delivery" arrived at its destination in Kentucky 45 days later, on Jan. 13.