Just ran my benchmark for Command (Op Bass Drum with missions added for US at 5x continuous).
Pre-patch - 28-29 game to real time ratio
Post-patch - 24-25 game to real time ratio
The pre-patch benchmark was run just a week ago, so its pretty current. The post-patch benchmark was run three times. So I would say its around 10% to 15% hit on performance. I am going to run the same benchmark on my older MSI and see what happens.
So the patching process is two step...
There's an OS level patch being released by all the OS manufacturers/ distros (your Microsofts, VMWares, Citrix, and Linux/Unix), but then there's also microcode that is going to be released by the chip manufacturers (this isn't just Intel, as AMD is releasing patches as well) more than likely as a BIOS upgrade.
The patching keeps the OS from exploiting the memory. The problem will be fixed with the BIOS upgrade, and that is where you'll see an impact.
Intel (and some of their partners) has communicated out that the increase will be seen on load based environments (Full Disclosure: I just got out of a call about two hours ago with one of Intel's server partners, and had a call with Microsoft yesterday about this specific issue). Where you'll see significant impact is on workload dependant systems - so Cloud hosting servers or physical database servers. The average user won't see an real impact, and its true impact is mitigated over time.