1. What type of update was it, Virus database or Program ?
If program, yes this is going to take longer than the incremental virus database update.
2. Was this update initiated Automatically or by the User, e.g. after notification of a program update available ?
Automatic updates have a CPU% restriction to stop the update using too much and throttling other programs.
A user initiated update after either a notification an update was available or a manual update have no such restriction. This is because it is initiated by the user who presumably wouldn't do so when they were busy.