Hello,
The conversion plugin transforms a document of 10 pages in about one second but this time may vary if the document has a lot of images and the server specs.
For example, this is the time that takes the document of 10 pages you sent to our contact email on one of our test servers:
real 0m2.105s
user 0m1.876s
sys 0m0.084s
There's no option to enable any kind of compression before doing the conversion.
One test you could try, as you're working with more than one DOCX and then you merge them to a single DOCX, is to transform each DOCX separately to PDF and merge the output using mergePDF method (http://www.phpdocx.com/api-documentation/docxutilities/merge-pdf-documents-with-PHP).
Regards.