Epistemic Status: Based on my first-hand experience. It is possible that I overestimate the strength of the effect, but I feel fairly confident that it exists.

I often observe, that when I do something that I find very difficult for some time, I get used to the difficulty. Importantly, "getting used to" seems to be a global attribute of my mind. When I switch to another easier activity, that activity seems easier compared to how easy it seemed, before doing the difficult activity. Normally easier also corresponds to the activity being more engaging. Probably because any progress you make seems to require less effort compared to the difficult task. This in turn means that you will make faster progress when working on the easier task than you would have otherwise.

I think of this as setting a difficulty anchor. You do something difficult, and if you do it for long enough then your brain is using the difficulty of that task to evaluate the difficulty of other tasks. The same effect seems to also apply to easy tasks. Playing computer games for very long will make other things seem harder because it will be harder to make progress/dopamine. After all most games are optimized for the player experiencing lots of progress/dopamine.

The difficulty I am talking about is the difficulty in terms of how hard you need to think. It is difficult, to never be late, or to shoot someone in the head in Counter-Strike (if the other player is competent). But that seems different. Never being late is mainly difficult because of things outside of your control. And once you are good at Counter-Strike, what your brain does might still be complex, but in some sense, it is not difficult. If you are competent, you already embedded the complex algorithms that are necessary to play the game in your neural network, and you can simply execute them. Only tweaking them slightly, depending on the specific situation you are in.

I am talking about the sense of difficulty, in which learning mathematics is difficult. Most of the time when you learn math (somewhat depending on your style), you learn new concepts, and how these concepts relate to other concepts. And this seems to strain the brain a lot.

Probably learning new things is one of the most difficult things for the brain. In my experience, learning how to draw, or learning a language, seem difficult in the same sense. However, at least for learning a language, once you get into it, and know what to do, e.g. because you follow some program, and know how to make progress within that program, it gets much easier. Much of the difficulty there, seems to come simply from the need to learn how to learn a language. Just going through flashcards and trying to recall the answers, and memorizing new cards does not seem that straining on the brain, at least compared to learning novel math concepts.

I am also not talking about the difficulty of doing something when you don't truly want to do it. Although that might have a similar effect. I am talking about things that are just intrinsically difficult to do for the brain, even when you really want to do them.

But besides the brain just needing to work hard, being demotivated by your results seems to have the same effect of anchoring your perception of difficulty. For example, when you are stuck on a math problem for a long time, your brain starts to drown your motivation. Not making any progress is not necessarily straining the brain though. But if you have been in that state of low motivation for some time, and you switch to another activity, where making progress is easier, then that task will be more engaging than it would otherwise be.

If my observation is correct and generalizes to other people, then you might be able to use this insight to boost your productivity. So I suggest that you should at least perform the experiment yourself, and see what you get.

I have found that mathematics works best for me, for setting the difficulty anchor. But that is simply because it is the most difficult thing (in the relevant sense) I know about. I have also found that doing at least 2 hours of math seems good. Not only does this seem to set an anchor for the rest of the day, but also doing less than 2 hours of math can be frustrating when you just spend almost one hour, in the beginning, to understand what is even going on. So if you just do one hour you might feel like you are not making any progress. Although that might set your difficulty anchor harder in less time, I would not recommend it. If you want to learn math, making it more frustrating than necessary seems bad. Just doing math normally is frustrating enough. Though I guess it depends on what and how you study.