Yes, the web interface has been slow since the beginning. Especially with actions on the volumes (list,create,modify,delete,etc).
But it has become unusable at this time: more than 2 minutes to list the LV on the VG, same time for every action.
It's weird to note that creating a LV with web interface takes 2 minutes but doing so with API takes 10 seconds.
As we're evaluating DSS, it's DSS V6 trial version.