The point of the thread pool (with a size of 2 by default) was to reduce the impact of the (very expensive) notification gathering on the XWiki instance.
The refactoring done in
XWIKI-17052 skipped that to only rely on async rendering. While it does not block http threads (but that's actually not true for notification count) it still produce way too much threads doing notification calculation. For example it makes myxwiki.org barely usable.