Dell has started pushing out the Android 5.0.2 Lollipop update for the Dell Venue 8 7840. You can check if the update is available for download from the Settings app.
Like the KitKat version before it, Lollipop on the Venue 8 seems to be unadulterated and close to stock Android. According to Android Police, it does enable encryption by default, which we have seen in the past causes some drop in performance, but it remains to be seen if that holds true for the Venue 8 as well.
If you have the device, check if the update is available.