There's quite alot of information missing for anyone to give you a meaningful response, so let me see if I can guide you into giving us what we need.

When you run both WoWs, look at a performance monitor (task manager if you're running Windows, for example) and tell us what you see.

Are you CPU-bound (CPU or any single core running at 100%)?

Are you memory constrained (physical memory available very low when runing WoWs)?

Do you lag all the time? Only when zoning / loading textures?

Are you running from a single folder, or multiple folders? A single disk or multiple disks?

A single monitor? Multiple monitors?

What OS?

Have you tried all the tips for improving WoW performance elsewhere on this site (maxfpsbk for example)?

What sort of system do you have? What CPU? How much memory? What GPU (although that's largely irrelevant for WoW so long as it's not a laptop with shared video memory....)?

If you provide at least some of the above then somebody will be able to help you. The first step in solving the problem is, of course, identifying it, and without knowing what's making you lag (which you'll get from the performance monitor) any suggestions would be a stab in the dark.