Creating a static sf::Text will gain some performance?
Maybe, but that's a workaround for a problem which doesn't exist when using idiomatic OOP. With static variables, you introduce new issues like destruction at global exit or race conditions in multithreading. Use
static variables if you want to have one variable per function, not as an optimization.
Anyway, "drop by 300 FPS" isn't a problem per se, of course it takes some time to draw text. Also note that FPS are not a linear scale. Without knowing how many FPS there were before, this statement is meaningless.
We should only investigate this as soon as it becomes a real problem, i.e. the application is too slow.