The speed is supposed to adjust to the amount of rain. The little dial button adjusts the sensitivity. If adjusting the senstivity button does not have any effect, or if the wipers don't automatically speed up or slow down based on the amount of rain when set in the auto setting - you may have an issue with the windshield itself delaminating. (This happened to me).

Windshields are two pieces of glass pressed together with a film in the middle to prevent shattering in case of breakage. If it starts to separate where the rain sensor is located, then it impacts the ability to judge how much rain is hitting the windshield. If that is the case, you would have to replace your windshield in order to fix.