Before every major iOS candidate and after all the Beta builds, the GM or Gold Master version is released which contains all the bug fixes and resolved problems from the previous betas.
iOS 5, the next major release is said to be released as the GM version on the 23rd of September (as rumors say). Well this could be true or not, but iOS 5 is actually going to be released pretty soon.
Expect a free-from-bugs, less laggy iOS 5 soon. :D