This isn't even taking front-end taking into account. The main HTTP response handled by PHP is taking 4 seconds to compose the initial response.
https://www.mediawiki.org/wiki/Manual_talk:Coding_conventions/JavaScript?forceprofile=1
100.00% 4171.235 1 - main() 99.38% 4145.466 1 - MediaWiki::run 99.38% 4145.279 1 - MediaWiki::main 93.61% 3904.854 1 - MediaWiki::performRequest 93.41% 3896.461 1 - MediaWiki::performAction 93.37% 3894.558 1 - Flow\Actions\FlowAction::show 93.37% 3894.547 1 - Flow\Actions\ViewAction::showForAction 93.34% 3893.543 1 - Flow\Actions\FlowAction::showForAction 92.95% 3877.242 1 - Flow\View::show 86.07% 3590.069 1 - Flow\View::buildApiResponse 77.17% 3218.802 2 - Flow\Block\TopicListBlock::renderApi 68.63% 2862.840 55 - Flow\Formatter\RevisionFormatter::formatApi 67.93% 2833.583 1 - Flow\Formatter\TopicListFormatter::formatApi 67.92% 2833.219 1 - Flow\Formatter\TopicListFormatter::buildResult 60.17% 2509.645 724 - Flow\RevisionActionPermissions::isAllowed 45.94% 1916.432 55 - Flow\Formatter\RevisionFormatter::buildActions 42.99% 1793.408 3021 - Flow\Data\BufferedCache::getMulti 42.84% 1786.768 3021 - Flow\Data\BagOStuff\LocalBufferedBagOStuff::getMulti 42.76% 1783.817 3117 - MemcachedPeclBagOStuff::getMulti 42.57% 1775.711 3021 - Flow\Data\BagOStuff\BufferedBagOStuff::getMulti 36.86% 1537.330 1247 - Flow\Data\ObjectLocator::findMulti 35.04% 1461.736 1238 - Flow\Data\ObjectLocator::find 34.12% 1423.096 1247 - Flow\Data\Index\FeatureIndex::findMulti ..
I can consistently reproduce this. Other non-Flow pages seem unaffected.