I assume there is no big difference if the file is almost empty or quite big… I’m not sure there is much to optimize. Depending on your system placing both Nextcloud and Collabora binaries on an SSD might help a little… Collabora is quite huge application integrated with pretty complex protocol (see Collabora integration guide for details). I think 2-3 seconds are good already for the first document. If you open multiple docs in fast sequence latter will open faster due to OS caching I bet.