The recent update of FTB DireWolf20 modpack (to version 2.3.0) included update of Chisel, from 0.2.0.31 to 0.2.1.34.

Though something must have changed (perhaps related to #730), because Chisel's speed boost on concrete blocks does not happen anymore.

Another player also reported this: https://minecraft.curseforge.com/projects/chisel?comment=553

For now, as a temporary fix to get the speed boost back, I have reverted to using 'Chisel-MC1.12.2-0.2.0.31.jar' in the FTB DireWolf20 modpack 2.3.0