I’m not a developer, but I work in tech. Interruptions are still quite costly for me. It’s less about “holding a lot in my head at once” and more about getting into a “flow”-ish state. Once I get going, I get momentum and am able to continue to get things done. Once I stop, I lose momentum and my pace of work slows.

It’s harder to quantify but when I get uninterrupted work done, I “feel” better while I’m working. I’m happier and more productive while working. I have a pretty one track mind, so focusing on one thing at a time and doing that one thing really well is important for me.