Here are my results of testing with and without the mitigations for Spectre variant 2 (branch target injection).
To recap, these mitigations have two parts: new microcode (which is like a patch to code hardwired in the CPU), plus a Windows patch.
New microcode is already available for some CPUs, but by no means all.
Without new microcode, Windows patches can address the Meltdown bug (rogue data cache load) and Spectre variant 1 (bounds check bypass), but not Spectre variant 2 (branch target injection).
So far, no new microcode is available for the i7-4790K (and ASUS Z97 motherboard) which I tested earlier. New microcode is available for this HP ZBook 15 notebook (which by the way is four years old, well done HP).
Here is what I found for the ZBook. Again this is making 15m contours for SRTM 1s DEM of the South Island of NZ in Manifold 184.108.40.206, using auto-generated SQL, running TRIM, then rebooting before each test.
(1) No mitigations
HP BIOS 1.37. Windows 10 version 1703 build 15063.786.
Baseline time 329.83s.
(2) Software mitigations only
HP BIOS 1.37. Windows 10 version 1703 build 15063.996.
Times 300.789s, 336.119s No significant difference.
(3) New microcode only, no software mitigations
HP BIOS 1.40. Windows 10 version 1703 build 15063.786.
Times 323.307s, 327.301s. No significant difference.
(4) New microcode, with software mitigations
HP BIOS 1.40. Windows 10 version 1703 build 15063.996.
Times 368.577s, 366.839s, 365.142s, 378.437s, 381.69s.
Comparing the average of times under (4) with the combined average of times under (1), (2) and (3) shows a 13% slowdown in performance.
This is only one machine, and only one kind of test. Other machines and tasks might (probably would) show very different results. But bearing that in mind, the slowdown in these tests is both consistent and significant.
For now I'm going to leave this machine fully patched and put up with the loss of performance.