Blackberry has updated the BBM Beta for Windows Phone app to version v126.96.36.199.
This build is focused on performance improvements, in particular addressing slow initial launch time on a fresh install.
Blackberry also said the the improvements will soon come to the stable version of the app, saying:
We’re very close to our next public launch of BBM for Windows Phone.
The BBM Beta app can be found in the Windows Phone Store here.