Shared Array Buffers are enabled by default in Chrome now, because Chrome has separate mitigations against Spectre.

To the best of my knowledge, it does not have mitigations against Meltdown because it assumes those protections will be implemented at the OS/firmware level, but if anyone has more experience or insight than me on that front, they're welcome to correct me.

In any case, you're making a kind of wild assumption that the type of user who disables a security feature from their OS to get a speed increase won't also likely disable security features like Site Isolation in their browser when they hear that those features increase Chrome's memory usage by somewhere between 10-20%.