The most likely cause is Lightroom itself, not the camera. Adobe introduced performance problems in some Lightroom CC/6 releases, including slower import/tethered behavior. Adobe later acknowledged these slowdowns and released an update that largely reverted to the older import code to improve performance.

So if the slowdown started only after the Lightroom upgrade, Lightroom is the primary suspect. The practical fix is to update to the Lightroom version that contains Adobe’s import-performance fix, or test with an earlier stable version if available. The OS upgrade may have coincided with the issue, but based on the reported behavior and community feedback, Lightroom’s changed import code is the more likely reason.