What factors influence conversion time when you upgrade a Microsoft Exchange Server 5.5 database to Exchange 2000 Server?
Database size, processor speed, and the I/O subsystem speed all influence the speed of upgrading the database. In general, upgrading a small database will be faster than upgrading a large one, and a faster machine will usually make the upgrade quicker. However, some surprises are hidden in the code. First, the process of converting messages is much faster than the process required to convert folders, which means that two equally sized databases—one with many folders, one with only a few—will have different conversion speeds. I've seen one estimate that says that nearly 90 percent of the conversion time is spent converting folder structures, which makes sense when you consider all the database objects and pointers that the upgrade process must convert.
In addition, one thread handles a large part of the conversion process, which ensures that multiple threads don't step on each other but also slows the upgrade process, even on large multiprocessor machines.
—Paul Robichaux