register-preview-delay

register-preview-delay is a variable defined in `register.el'.
Its value is

1

Documentation:
If non-nil, time to wait in seconds before popping up a preview window.
If nil, do not show register previews, unless `help-char' (or a member of
`help-event-list') is pressed.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.