Going from 1 core to 2 hyperthreads was a big bonus in interactivity. But I think it was easy to get early systems to show worse throughput.

I think there's two kinds of loads where hyperthreads aren't more likely to hurt than help. If you've got a tight loop that uses all the processor execution resources, you're not gaining anything by splitting that in two, it just makes things harder. Or if your load is mostly bound by memory bandwidth without a lot of compute... having more threads probably means you're that much more oversubscribed on i/o and caching.

But a lot of loads are grab some stuff from memory and then do some compute, rinse and repeat. There's a lot of potential for idle time while waiting on a load, being able to run something else during that time makes a lot of sense.

It's worth checking how your load performs with hyperthreads off, but I think default on is probably the right choice.

Definitely measure both ways and decide.

For many years (still?) it was faster to run your database with hyper threading turned off and your app server with it turned on.