In order to make the compression level configurable, the following property is added to the configuration file infostore.properties:
# Configures the used compression level that is applied to .zip containers
# when downloading multiple documents at once. Possible values are -1 for
# the built-in default level, 0 for no compression, or any number between
# 1 (best speed) and 9 (best compression). Defaults to -1.
com.openexchange.infostore.zipDocumentsCompression Level=-1