On various Vector feedback pages as well as on OTRS many people report that since the switch to Vector it takes significantly more time for Wikipedia pages to load.
I didn't think that i experience significant slowness myself, but after seeing a few reports i tried to measure it in a very non-precise way: I loaded [[The Holocaust]], a long article with a lot of images. It took 25 seconds with Vector and 15 seconds with Monobook. I did both measurements after cleaning the cache.
Some other articles:
- [[Slavery in the United States]] - 13 sec. on Vector, 7 sec. on Monobook
- [[Jehova's Witnesses]] - 10 sec. on Vector, 6 sec. on Monobook
- [[Nuclear program of Iran]] - 14 sec. on Vector, 9 sec. on Monobook.
Seems quite consecutive, but i'm not a professional website performance tester.