I appreciate what you’re saying, and I do hate being slowed down in the moment.

I’ve got a handcrafted AutoHotKey script and a 22 button gaming mouse fine tuned for 2D CAD / CAM software at work, so while I am definitely in to this sort of customisation...

If I’m honest with myself I don’t really get more work done because I end up using the time being unproductive elsewhere.

There’s definitely a certain intellectual curiosity-satisfaction to be had from customisation.

I’m just not convinced it’s a net time saver in the long run?

Isn't that literally the point? If you can do your task in 10% less time, and then end up spending that 10% procrastinating on HN, then you still win - the task gets done, and you also get to sneak in something nice. And if you find you'd rather spend that time on something else, or doing more work to finish everything faster, you can choose to do so. Optimizing work gives you choice what to do with the time saved; the default may be that you waste it, but now you have options.

Ah, yep, you might have a point there!

Dunno how I missed that. Seems obvious now.

User interface design (UID) and user experience (UX) are interesting topics.

Have you ever conducted an oil change at your car or some other minor mechanical task? Wasn't it fun for a moment (at least that one time)?

Machines are interesting.