Web designer freezes when adding event handlers on pages with many controls

Adding event handlers to a Web control through the Add Event Handler dialog is extremely slow on pages with many controls and event handlers: selecting all of a RadioButtonGroup's events takes around a minute of frozen UI on a fast machine, and merely opening such a project takes several seconds. Performance degrades with the number of controls, their events, and existing handlers, making busy Web pages effectively unusable in the designer.

Expected: the IDE stays responsive; adding a batch of handlers completes near-instantly.