All Sony Ericsson hsets run slow unless you leave them on the Sony Ericsson Homescreen
Change the Homescreen to Sony Ericsson (Menu > Settings > General > Homescreen).
Also if necessary download the latest SW from the www. sonyericsson.com website, also ensure that all txt msg are deleted from all areas of the hset, as this is also a known cause.
Good housekeeping of the Txt Inboxes etc does offer improvement.