Select the target folder locally
When you start an export, MyDataMerge will ask you where to save the resulting files. Choose a local folder – not a network folder, not any folder located on the desktop (or as subfolder). There’s a known macOS bug slowing writing operations down when exporting to desktop.
Stop Backup engines
Stop the time machine and any syncing services (e.g. dropbox, google drive) when exporting to a folder being synchronized with a cloud.
Exporting large amounts of documents
When you need to export large amounts of documents use the export range feature and do it in chunks. Depending on your machine speed you might have to experiment a little on the size of a chunk to get the best export speed. An important part here is that your layout is well prepared.
Writing single files is slower than writing files in chunks
If you do not need single files at the end, and chunked files are ok for your post processing, try to use the “Custom records per document” option. So instead of exporting 2000 single PDFs export 20 PDFs with 100 records each. This is a lot faster.