What is the maximum known air speed of a dirigible vs. the lowest possible air speed for a [stealth] fighter?
Airship fastest speeds
According to Guinness the world record speed for an airship was 70 mph by Steve Fosset in 2004.
Guinness also say
The large rigid airships built by the USA and Germany in the 1920s and 30s could reach higher speeds (up to 140 km/h or 87 mph according to some sources), but these were never officially measured to FAI standards.
Stealth aircraft slowest speeds
The F-117 has a landing speed of 180 mph.
The F-35B is stealthy and can hover.