Known about for over a year: https://github.com/magento/magento2/issues/5880
Yet update to 2.1.8 on a server running PHP 7.1.8 and Magento dies. 2.1.7 is OK. And despite a huge list of fixes, the only one I needed, getting cron to import currencies, wasn't mentioned and is still broken.
However, to show I'm not a total moaner I do think its snappier. Fortunately the production server is PHP 7.0.22, but the time it saves by running faster is still wasted by me having to update the currencies manually from the back end each day.
