Earlier quoted context omitted.
Your web browser runs untrusted code. On a computer that never runs untrusted code you can do that.
There have been more Chrome/Firefox 0-days than speculative execution vulnerabilities exploitable in Javascript (0). Sure, there is a chance that the Chrome/Firefox teams missed something, but there have been sighted no exploits since the release (of spectrev1) and the browser fixes. It's not a crazy threat model to have on a personal PC, the risk is so very minimal. If your threat model is that strict you shouldn't…
Session keys, private keys, passwords and all other kinds of access tokens that your system is using, it's the next worst thing from remote code execution.
Your browser runs so much untrusted code that it's really unreasonable, and yes, we should definitely be pushing back hard on this. But it's probably the most stupid thing you can do to disable these mitigations because they're not just theoretical, they're real, they're here and everyone knows about them.
This is like anti-vaxx philosophy. "the risk is low"; well, maybe the risk is low because of herd immunity, it's not feasible to run these attacks as they'll be obvious to those who have the mitigations in place (100% CPU), but if there's a 0.00000001% return then it becomes profitable to exploit, just like mail spam.
Do not fucking turn off these mitigations on desktop computers, they are too complex and run untrusted code all the time. Unless you can work without javascript and I doubt you can because the web today is basically unusable without it.
If you have a database which is only accessible internally, you can disable the mitigations, because those things are hit hardest by the mitigations and do not run untrusted code.
But really, your desktop is running untrusted code _a lot_. Please do not do this, not only for your own sake but for everyones sake. Don't make it profitable for malicious agents to run these attacks.
[0]: https://www.reddit.com/r/javascript/comments/7ob6a2/spectre_...