OutputTransformPipeline runs various post-cache transformations on article HTML. On non-parsoid wikis, this consists of deduplicating style nodes within the content in addition to ToC handling for mobile output.
These transforms are expensive for larger pages. On mwdebug, the desktop transforms for the Barack Obama page take 350+ ms on a parser cache hit, twice as long for mobile which runs an additional step.
As of May 12th, we currently spend ~10% of index.php wall time in these transforms (4.5% overall). While T394005 will offer a long-term solution once Parsoid is the default everywhere, it may be worth improving the existing transforms to limit the impact.
