If the item is coming from within the EU customs examinations are almost non-existent. (EU law requires the free movement of goods between all 27 member countries but packages might still be sniffed by dogs looking for drugs).
If an item is coming from outside of the EU then it needs to be examined by customs staff (or, more accurately staff who are working on behalf of HMRC) to assess whether it's liable for excise duty, import duty and/or VAT. The sender should have attached a CN22 customs declaration form to the package. (If not, it can be seized without any compensation). The CN22 form should state the nature of the contents and their value. The people examining packages sometimes simply trust that information but they'll frequently open packages to check the contents.
Inspecting packages (and calculating the bill you'll have to pay, if appropriate) sometimes adds a day or two (or several days at peak times) to delivery times.
Chris