Consolidating duplicate fonts takes ages and Acrobat crashes at the end

Dear all,

I am trying to combine various PDFs via Acrobat XI Pro into a single PDF file. While at the beginning the process works more or less smoothly it gradually gets more time consuming (consolidating duplicate fonts) the more pages the final PDF file contains (>3000 pages). This leads eventually to the point that integrating a one page PDF file (via "combine files into single PDF" or "insert from file" it does not matter) takes up 1 to 3 hours just to crash at the end completely. The problem occurs with many different PDFs I am trying to combine and not one specific file.

Here the technical specifications:

Acrobat Pro XI: 11.0.17, Architecture: i386, Build: 11.0.17.9, AGM: 4.28.147, CoolType: 5.11.145, JP2K: 2.0.0.36554

MacBook Pro: 2.8 GHz, 8 GB from 2014

OS X: El Capitan, 10.11.6

Any suggestions would be highly appreciated.

Thank you in advance

DJ