Here is the same, the moment things get too much, it start hallucinating and missing important things. It also depends on what model you are using. I read that Gemini 3 pro, which has limit of 1 million tokens can decrease its productivity to 25% getting close to its limits. Not WITH 25% but TO 25%. Becomes extremely dump.
Other models are just asking too many questions...
There are some tips and tricks that you can follow. And it is similar to how people work. Keep the tasks small, save what the model learned during the session somewhere, and re-use this knowledge in the next session, by explicitly writing to read that information, before it starts.