Often the difference is between small personal projects where performance can easily be kept under control, whereas large production products, that have to hand

Often the difference is between small personal projects where performance can easily be kept under control, whereas large production products, that have to handle all the edge cases, accessibility, work on many platforms, etc etc can be quite a bit more complex, require more people working on it, and performance gets harder to keep under control. That last 10% to can be 90% of the complexity and work. Text editors, especially with language aware features, are far from simple.