Type the following commands:
gsettings get com.canonical.Unity.Lenses home-lens-default-view > broken_scope.txt
gsettings set com.canonical.Unity.Lenses home-lens-default-view "['applications.scope', 'files.scope']"
Now you have an absolute minimal lens scope (only applications and files; no music, no videos, no pictures, ...) (you might have to log off and log on again)
If that corrects your problem and you're happy to continue like this, leave it alone as I'm running like this and the dash is very responsive! >:)
If you want to add some of the other stuff back, open broken_scope.txt
and add them one by one to the second command above, and test whether the problem comes back. If it has, remove the last one and continue with the others...
gsettings list-recursively | grep --ignore-case tomboy
(if any)