I tested latest versions of Plantuml (1.2019.6 and 1.2019.8) and I noticed that the preprocessing (by using -preproc flag) step taking lot of time when compared with old versions like 1.2019.1 and 1.2019.4.
Here are the stats to preprocess 2500 files approximately.
With versions 1.2019.1 and 1.2019.4 - Just 25 secs (0.4 mints)
With versions 1.2019.6 and 1.2019.8 - 480 secs (8 mints)
I have tried with Beta also http://beta.plantuml.net/plantuml.jar but with the same issue.