I am working on improving website performance. Here are the list of key performance indicators I am looking at for each page
Is there any optimum value for each KPI to indicate website performance? Please help me in this regard.
Research on a wide variety of hypertext systems has shown that users need response times of less than one second when moving from one page to another if they are to navigate freely through an information space.
This quote is from The Need for Speed by Jakob Nielsen that was written in 1997 and it's still valid today.
If you can serve pages in a second or less (the whole package: DNS lookups, processing time, assets loading), that's ideal. In real life not many websites can boast this kind of speed. However, you can get pretty close by improving frontend performance of your websites if you follow guidelines from people at Yahoo and Google.
Yahoo have their YSlow Firefox plugin and a companion guide explaining all of their Performance Rules
Google has a rival Page Speed Firefox plugin and corresponding Performance Best Practices.
This should be a good start. These guidelines don't provide specific benchmarks for every point in your list (but they do provide some) - the goal is to minimise the loading time and size of each and every request to get as close as possible to "instant" response time of less than a second.