I had bought stuff from the U.K. and China in the past..... depending on the shipper and the shipping method. Some came fairly fast, the seller shipped the same day and once the package hits N. America, it was carried by the local carrier. Some came very late, shipper did not ship the same day, package traveled by land then by sea, then once it hits N. America, it was bouncing around between several carriers before arriving.
It depend on the method of shipping and whether the origination is close to the hub where the shipping is routed.
If it takes several days to reach the shipping hub then, those will be part of the delay.
It also depend on whether the U.S. Custom will select the package for inspection from profile or random, that could be another day to 5 days delay........