It'd be nice to be able to easily boot into, or toggle into, a performance optimized, disabled mitigations environment to do something while offline.. many computer uses don't require being connected to other computers. I've gotten into the habit of hotplugging my Ethernet connection, personally.
You can actually do that fairly easily, just add the parameters linked to a second boot entry in GRUB.
However, I would very much not advise doing so, as I still am unaware of any task that can both, be done without the need for a network connection, while also being significantly slowed down by the mitigations, after recent improvements to the kernel and software. Basically, the potential benefit is very low in a lot of tasks, whilst requiring additional security measures (ideally fully air-gapped) and that you reboot the system every time you'd do such a task.
Also note that, in theory, just being temporarily offline may not shield from being exploited fully.
As an example (the only case that I've identified personally), if your curious, I have a (windows; Intel q6600) box that I use for gaming occasionally. Single player game I like, Total War: Shogun 2, runs at about 55 fps (benchmark) pre-Meltdown/Spectre/etc. Now it gets ~22 fps. I can use https://www.grc.com/inspectre.htm to toggle some mitigations to get it playable again.
