Terrain Brush shortcuts cause strong lag

Hi,
I'm using the new Terrain tools, everything seems to work fine no lag during terraforming operation.
However, each time I use the shorcuts for brush (hold A, S, D), it lags and takes ages to detect the key up, so I'm stuck with the adjust settings for 5/10 seconds each time.

Someone else met the same issue? It's not usable as is, and editing terrain without these shortcuts is really slow and painful.

Thanks for your help.