org-use-fast-tag-selection

org-use-fast-tag-selection is a variable defined in `org.el'.
Its value is

auto

Documentation:
Non-nil means use fast tag selection scheme.
This is a special interface to select and deselect tags with single keys.
When nil, fast selection is never used.
When the symbol `auto', fast selection is used if and only if selection
characters for tags have been configured, either through the variable
`org-tag-alist' or through a #+TAGS line in the buffer.
When t, fast selection is always used and selection keys are assigned
automatically if necessary.

You can customize this variable.