I installed LR6 and found the performance tab (Edit Preferences) was there and the GPU was checked.
Then I installed a new graphics card and now the Performance tab is missing.
Replaced an EVGA 650Ti with an EVGA 970 SC card
Bruce in Philly
Solved: backup your catalog, close LR, restart LR resetting your preferences:
Hold down [Shift][Alt/Opt] and Launch Lightroom
Accept the preferences dialog.
Retrieving data ...