My friend Jennifer is an avid photographer and switched to Lightroom about a year ago.  She came by because she said that things were horribly slow and that she had convinced her husband to get her a new Mac, but what could be done in the interim? I checked all the usual suspects and when launching and exiting Lightroom 5.7 with my usual optimize / backup checked, I had to kill the process after SEVEN minutes.

She had just under 8,000 images in the current catalog and it was over 3.2GB in size.  I went over to my Mac Pro.  My current in use catalog has about 32,000 images in it.  My catalog was under 500mb.  What?

Jen loves to experiment.  She'll try presets, and plugins and sliders to learn what they do and to push her creative bent.   I made a discovery that surprised me.  Lightroom saves your History into the Catalog.  I mean it has to save it somewhere, but I never thought about the "where".

So I did an experiment after checking that there was a solid backup of the catalog and confirming with Jen.  I went to the top of the Lightroom Catalog in the Library module and did an Edit | Select All to pick every image in the catalog.  Then I switched to the Develop module and from the Develop menu chose Clear History.  A dialog box popped up to ask if I wanted to delete the history for the current image or all selected images.  I selected all and then sat back to wait.

=== After I posted this, a comment was made that I missed a step ===

=== Not so ===

Lightroom did the work in the background, and in about five minutes, the Catalog had dropped to 1.8GB in size.    Exiting Lightroom was fast this time, exiting with a full backup and optimization in under two minutes.  The final catalog was even smaller after optimization.

So, if you find Lightroom is dragging, and your catalog file seems disproportionately large, perhaps consider clearing your history.