We might all be operating under different ideas of what "matter of hours" means. Often times in a project, at least for me, the range of things that I can start and finish in mere hours is not actually infinite, but rather constrained. Only the simplest and smallest things can be done so fast or faster. So many other things just take longer, at least a day and a half, and can take extra long when other teams are involved. More experienced developers can learn how to break things down into increments, so while an entire big feature might take weeks to do, you're making clear chunks of progress in much smaller units. Those chunks are probably not mostly sized in mere hours pieces, though...

You're right that priority matters. Just beware of priority queue starvation. Still, if some newly discovered bug isn't urgent, even if I think it'd be rather easy (under an hour) to fully address, I'd rather not break my current flow, and just keep working on the thing I had earlier decided was highest priority. A lot of the time something will prevent direct progress and break the flow anyway, having smaller items available to quickly context switch to and finish is a good use of those gap times.

The "DB handling 100x the traffic" example above isn't quite well defined. I wonder if it's making queries return 100x faster? Or is it making sure the queries return at roughly their current speeds even if there's 100x more traffic? Either way, I can make arguments for doing the work proactively rather than reactively, but I'd at least write down the half a dozen things. Then maybe someone else can do them, and maybe those things can be done in around half a dozen tiny increments of 30 minutes or less each, instead of all at once in hours.

Exactly. Going for something that shaves 5% off your hosting bill or your build time only makes sense when you are already colossal. But something that halves your bill, or halves your build time, or halves your latency, is likely impactful if your project achieves the scale when the proceeds from it can support you.

And very often an optimization improves things not by factor of 1.1 but by factor of 10 or more.

OTOH it's worth to be mindful of your "tech debt" as a "tech credit" you took to be able to do more important things before it's time to repay. Cutting your hosting bill from $100 to $50 may be much less important than doubling your revenue from $500 to $1000.