It will be a big performance change for dapps using HAF, and I hope that before long most new dapps will be written using HAF (the performance benefits are substantial for any dapp that wants to scale).

But for existing dapps that just directly process the head block, you're correct: it won't speed them up, it'll make them more resistant to failures in the face of forks (arguably more important than making them faster, of course).

It also wouldn't surprise me if some existing apps wait a block or two (IIRC, hivemind does, for example).