The reason why people thing "Even if you do, you are only going to get a few percentage points." is because it generally takes 5-50x the developer time to optimize such code. If it takes half a day to write naive code to do something like validate utf8, it probably takes ~25 workdays to make a fast SIMD version. If you instead spend an extra half a day, there is a good chance you get a 10-50% speedup using normal code.
And most of the speedup is because of tricks to avoid doing beaches. There is a great blog post from one of the authors of JSON SIMD discussing this.
I'm on mobile, there is a link for the blog post on the simd JSON github repository.
The blog post I mentioned:
Paper: Parsing Gigabytes of JSON per Second https://branchfree.org/2019/02/25/paper-parsing-gigabytes-of...
Another related post from Lemire:
Ridiculously fast unicode (UTF-8) validation https://lemire.me/blog/2020/10/20/ridiculously-fast-unicode-...
Those algorithms are fast. But to put them in perspective. A single x86 CPU can write 64B per cycle. At 5GHz, the theorical maximum bandwidth is 320 GBps. IIRC, the read bandwidth is twice that.
There are others botlenecks, and is very hard to write code that writes at every cycle.
A interesting consequence, is that the theorical maximum bandwidth is logarithmical to the number of cycles. Again, talking about branchless streaming application.