Hammer/Mace & Shield’s bash is slower than any other shield’s bash.
You can easily test it. Take IB with Miner’s Rythm talent. Use the combo push-bash vs a dummy in the keep.
Miner’s Rythm bonus lasts 2 second and it refreshes at every heavy attack (bash included)… so, sooner you see it refreshed, faster the heavy attack (bash) is.
With some test you can easly notice that Hammer and Shield’s bash is really slow.