There is no lag with Dev Preview and I can confirm that, how? Because I am using it on Lumia 520, Lumia 925, Lumia 720 and Lumia 1020. None of these lag, at all.
What you need to do is hard reset, after setting up the preview. Why? Because system files between WP8 and the preview aren't exactly the same. The installer is meant to add and remove files automatically but if you have used a PC before, you would realise it isn't always as easy as that. By performing a hard reset, you keep all the new files, you get rid of old temp files that installer may or may not have removed and you remove all old apps. DO NOT restore a back up if you want to see how optimised the Dev Preview is. Start fresh, install apps that are compatible with WP8.1
If you install whatsapp, facebook, twitter and Skype and think they aren't doing what they should do - they wont. Why? They aren't updated for this OS. They work best with WP8.0 and won't take advantage of any features that WP8.1 has to offer in the preview.