Here is my un-scientific test results. I imported and exported the same files with both programs. There were 201 files. With LR5.7 the import time was 5 min. 27 sec., with LR6.6 it was 8 min. 49 sec. The export time for the LR5.7 files was 4 min. 19 sec. and 5 min 0 sec. for LR.6. The import time was significantly longer with LR6.6 and export time slightly longer. I have resorted back to LR5.7 because I typically import 1000 to 1200 files at a time. I wanted to use some of the new features with LR6.6 but simply don't have the time. Shouldn't newer versions / upgrades be better than its predecessor?
Have you tried turning off the Graphics Processor from the Lightroom 6.6 preferences.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor
What is your system OS?? Where are you importing from and to where??? Same with export??