In my experience, not too important, except that if you have extended thinking on it makes things a little slower.
Also, often you can make your system prompt shorter by just replacing phrases with shorter ones and seeing if it has the same effect, etc…
The way I do things is my first system prompt is always a long list of everything I think I want, and then after time I can prune it down and delete all my inevitable repeats.
In my experience, not too important, except that if you have extended thinking on it makes things a little slower.
Also, often you can make your system prompt shorter by just replacing phrases with shorter ones and seeing if it has the same effect, etc…
The way I do things is my first system prompt is always a long list of everything I think I want, and then after time I can prune it down and delete all my inevitable repeats.