Does the IOS 8 is slow in the iPod touch 5g ?

Nobody here who knows can answer your question. iOS 8 has not been released to the public and the people who now know can't post in a public Apple forum like this one.

If you are a developer using iOS 8, you need to move this discussion to the Developer Forum.

Otherwise, iOS 8 has not been released and is not available for discussion yet.

IOs8 has been released now but don't download it - unless you have a couple of days to look something up on safari