For more detailed information: https://www.gov.wales/introducing-20mph-speed-limits-frequently-asked-questions
They’re not just lowering the speed limit. From the FAQ:
Will the roll out involve money being spent on speed bumps?
There is no plan to include traffic calming (including speed bumps) as part of the change to speed limits. There are other ‘softer’ measures that might be introduced, such as using buffer speed limits, removing the centre line, narrowing the carriageway visually, using planting etc.