Starting with WorldServer 11.6, a caching mechanism has been implemented to drastically improve TM lookup times.
This article explains the new functionality in detail.
Moreover, note that you can disable the Cached TM feature by configuring the Segment Asset step if required, as described in the documentation. The Cached TM functionality has no impact whatsoever on Live TM mode. You will still be able to update your TM upon each import of a return package or Save action. It is an additional feature to improve your user experience.
We strongly recommend watching the presentation about the new features introduced with version 11.6. in our RWS Community page. Starting from minute 9:00, the new export capabilities are explained.