It's utterly
impossible for Haswell to be any slower than Ivy Bridge. From Core 2 up to Ivy Bridge, Intel has used 6 execution ports. With Haswell, they're adding not one, but
two more execution ports!
They wouldn't do that unless they're convinced they can achieve up to 33% increase in peak instruction throughput. In fact for some workloads which are bottlenecked by instructions that require execution port 0, performance could in theory double since the new execution port 6 can take most of the same instructions. It will specifically be a big boost for software that takes advantage of Hyper-Threading. So the 10% number that is floating around, is quite conservative.
Also they added AVX2 support, which
doubles both the integer and floating-point vector throughput (for multimedia and game physics and such). And on top of that Haswell features TSX for
much improved multi-core efficiency, and the internal bandwidth has
doubled to keep this number cruncher well fed. All of this has been achieved without compromising anything.
Anyone spreading rumors about Haswell not being substantially faster, is making a fool of himself (either out of ignorance or to get rid of Ivy Bridge overstock).