I've only glanced over that paper, but they're using a processor with a 5-stage pipeline, which is really short by modern standards (Zen 1 uses a 19-stage pipeline, and I couldn't quickly find the number for subsequent versions of Zen). Using a very short pipeline significantly reduces the advantage of control flow speculation (CFS)... but they still showed CFS offering up to a ~50% advantage over their best alternative, if I'm reading that right.

I wish they had included the geometric mean of their benchmarks, but I didn't see it anywhere, and I'm not going to run the numbers right now. Even if the speedup CFS offered on a 5-stage pipeline is "only" 25%, that is still huge... and on a larger pipeline, that delta would grow. "Not much" is drastically different from my interpretation of those results.

I do think security is extremely important, but I'm not convinced that things are currently so terrible that this is the only way forward, as the authors seemed to imply.

OTOH, I would enjoy seeing a return of an Itanium-style ISA that moves a lot of speculation from the hardware to the compiler. I think compilers are in a much better place now than they were when Itanium hit the scene, which did not help Itanium's problems.