From the documentation string, it looks the custom variable impatient-mode-delay should be a number but it is set as a boolean:
(defcustom impatient-mode-delay nil
"The delay in seconds between a keypress and the browser reload.
Set to nil for no delay"
:group 'impatient
:type 'boolean)
From the documentation string, it looks the custom variable impatient-mode-delay should be a number but it is set as a boolean: