If we manually do this?

[GreenGecko]

Optimising your images manually is a good idea and can make a fair difference. CSS/JS less so, as text files can be compressed ( images are already as compressed as much as possible ), usually by a factor of 10 or more, so it's rare that the amount of traffic will be changed that much by it.

You would do better by posting an example WPT result, as the performance problems are often caused by a lack of server resources, especially with sites hosted on shared servers.