I use a GitHub enterprise instance, and we have enough server power to spare. There's no option to say "we have the server capacity, please render large diffs."

They probably didn’t tell the engineers who designed and implemented it that it was a cost–cutting measure. But even if they did, every option or preference that you add to your software multiplies the number of cases that you have to test. In principle a boolean option doubles the number of tests you need to do, because you have to run all of your tests with it off and all of them again with it on. Of course in practice people usually just assume that there won’t be any unwanted interactions between most of these type of options, which is often true enough. It is quite common to limit the number of such options in order to control costs over the long term. It reduces development, QA, maintenance, installation, and support costs. On the other hand, it does annoy users.