I am using google hosted javascripts libraries,(jquery ,jquery-ui & other google jsapi),and I noticed that these scripts can be accessed by both http & https schema.Now ,I want to know that what are the effects of using http or https schema to access these google hosted scripts , and for my projects ,it's just an ordinary websites ,using http as default schema, so ,what should I do ,http or https? Is there any performance issue between the two ?
https does affect performance negatively, as encryption and security negotiation aren't trivial tasks. In the vast majority of cases this performance cost is not significant enough to outweigh its benefits.
Remember that SSL also secures the identity of the web server and not just the channel.
If a "man-in-the-middle" spoofed the address of your script's location (for instance), https would prevent you from unknowingly executing unintended scripts. http would not.