There are two different issues...
5.0.15 is über-crappy in CalDAV for Lightning/SunBird (while it works nicely for iCal).
If you have calendar with lots of objects, you _need_ to upgrade to 5.0.16 to get a "normal" behaviour in Lightning/SunBird.
See here :
Bug 37044 : CalDAV with Lightning "broken" in 5.0.15
As you've tried with SunBird and found the performance unacceptable, I guess you're hitting this issue...
And you need to ugprade to 5.0.16 to get through it.
For people looking for the patch, the easiest way to find it is to look at Mozilla's bugzilla...
https://bugzilla.mozilla.org/show_bug.cgi?id=390036