Google Maps calculates journey times based on the speed limits along the route as well as live traffic information, and will constantly recalculate along your journey. For the most part changes to speed limits, road layouts and names are reliant on the relevant local authorities providing updated information, so it could be a matter of days or several weeks before this information is corrected. However, a reduction of 10mph will have a modest impact on journey times - a two-mile journey at 30mph will take four minutes, while the same journey at 20mph would take six minutes. Even if the calculated speed limits are incorrect, Google Maps will recalculate your arrival time based on your actual speed.