How far is my Time Machine "preparing backup" step?
If the CPU goes over the roof, as shown by Apple Activity Monitor (mds–stores) or MenuMeters (see below), Time Machine "Preparing backup…" takes forever when it should be done in seconds or similar, and Apple Mail and Finder Spotlight (File - Find) find nothing, chances are that Time Machine is checking the Spotlight FSEvents database to see what has changed since the last backup, and that that such database is corrupted or there is some other problem. The fix is:


Stop Time Machine backup clicking the "X" on the screen capture above.
Reboot the Mac.
If required, stop again Time Machine (if it takes forever again) and once stopped, select "Time Machine - Back Up Npw".


In my case, that worked like a charm on macOS 10.12.6 (16G2136) Sierra using internal booting SSD 2TB from Apple (Samsung) on iMac 27-inch 5K Retine (mid 2017) and external Time machine Samsung Portable SSD T5 2 TB. Time Machine takes about 15 seconds to perform each backup there!


If not fixed and Time Machine takes forever again, rebuild Spotlight index selecting "Apple - System Preferences - Spotlight - Privacy". Then drag and drop the full booting disk (or the one that is backed up) there. Then remove it to allow Spotlight to rebuild its index. Note that it may take from a few minutes to many hours, depending on the items stored in the disk and the disk speed (SSD is much faster than the mechanical rotational ones). You may want to do it overnight.


Finally, it is a good idea to install MenuMeters to easily see when the CPU goes over the roof…
MenuMeters for OS X El Capitan 10.11 (and later)
https://member.ipmu.jp/yuji.tachikawa/MenuMetersElCapitan