When you update X-Plane, it scans about 10,000 files before it runs the update. This scan checks every single byte of every file. As a result, it's a little bit slow - it takes a few minutes.
With the new installer, when you add and remove scenery, we do the same scan on the installed scenery. This scan is very slow - 78 GB is a lot of data!
When the next installer comes out (2.05, which will be cut after 920 goes final), the add-remove scenery function will do a quick scan to see what files exist. This is a lot faster - even with full scenery the scan takes only 10-20 seconds on a slower computer.
I can't pull the same trick for updates - we need to detect any possible defect in a file during update to ensure that the install is correct!