Looking through this post by Phoronix, it would seem that Intel had a significant regression in performance on Ubuntu 14.04 with the Linux 3.13 kernel. In some tests, HD 4600 only achieves about half of the performance recorded on the HD 4000. I have not been following Linux iGPU drivers and it is probably a bit late to do any form of in-depth analysis… but yolo. I think the article actually made a pretty big mistake and came to the exact wrong conclusion.
Let's do this!
According to the article, in Xonotic v0.7, Ivy Bridge's Intel HD 4000 scores 176.23 FPS at 1080p on low quality settings. When you compare this to Haswell's HD 4600 and its 124.45 FPS result, this seems bad. However, even though they claim this as a performance regression, they never actually post earlier (and supposedly faster) benchmarks.
So I dug one up.
Back in October, the same test was performed with the same hardware. The Intel HD 4600 was not significantly faster back then, rather it was actually a bit slower with a score of 123.84 FPS. The Intel HD 4000 managed 102.68 FPS. Haswell did not regress between that time and Ubuntu 14.04 on Linux 3.13, Ivy Bridge received a 71.63% increase between then and Ubuntu 14.04 on Linux 3.13.
Of course, there could have been a performance increase between October and now and that recently regressed for Haswell… but I could not find those benchmarks. All I can see is that Haswell has been quite steady since October. Either way, that is a significant performance increase on Ivy Bridge since that snapshot in time, even if Haswell had a rise-and-fall that I was unaware of.