I do development on a very large LTI provider project using Moodle and the LTI provider plugin that just got merged in to core. By large, think 600k+ tools, 8k+ courses, and we're only to the pilot stage of the project. In a year we'll have tens of millions of tools and over 100k courses.
Given the existing setup for the LTI provider cron which hasn't changed since it was merged with core, it could easily take an hour to run the cron with all of the queries it would be running, and talking to other systems.
We needed to be able to send grades back much more quickly than that, and came up with modifying core a tiny bit to get it done. Since the plugin is now part of core I've submitted what I used in the tracker so other people can make use of it, and possibly it will find it's way in to core at some point.
If you would find it helpful you can see it on the tracker here: https://tracker.moodle.org/browse/MDL-54809
If you want to see it in core don't forget to vote.
If you're on 3.0.3+ (Build: 20160317) feel free to use the attached version of lib/gradelib.php to get these changes. Only do this if you are only using Moodle as an LTI provider though, because any grade that gets updated will try to send back an LTI grade.