The speed at which your USPS parcel is delivered will not depend on whether it has a USPS tracking number but instead the USPS postage service you opted for or purchased (or which was purchased by the company or marketplace you ordered the product from). The cost of that service will depend on the following:
Size of the parcel: The size of a USPS size could mean it is shipped using different transportation methods, with larger and/or bulk shipments being sent by sea, which takes a lot longer than by air.
Weight of the package: The weight of your USPS parcel could also mean a variety of different shipping methods.
USPS service: Depending on the price you are willing to pay, there is a number of different USPS shipping services available which offer a variety of delivery turnarounds for customers.
Holidays: Holidays usually mean a larger amount of goods being bought and or sent which could affect the delivery time.
Natural disasters: Of course, sometimes unforeseen circumstances could affect the delivery of your package, which cannot be accounted for by couriers or merchants.