There are a two main factors that cause these delays:
1) Getting the files from the developer: If the dev is responsible, this step is quick. If the dev isn't...there's not much GOG can do about it.
2) Repackaging the files for GOG: - In this time, GOG's internal staff decides if the patch would be released in its own installer or if it should recompile a full installer for the newly patched game...or both. This process doesn't take more than an extra day, assuming patches don't release after-hours. If they do, things would only be delayed for, at most, three days.
So, most delays are not GOG's fault. However, GOG is still taking steps in order to streamline this process as much as possible with the soon-to-be-released Galaxy client...assuming you yourself decide to install and use it. The process may never be perfect, but if we are dealing with a dev that doesn't prioritize GOG as much or more as Steam and other vendors, there's not much that can be done.
DISCLAIMER: I am not, in any way, implying that Red Thread Games is doing any such thing. My statements above are just a general example and do not refer to any developer in particular. Thank you.