Is loading images over HTTP a good idea to improve performance?

Solution 1:

No, this is a bad idea, as this is not secure. We call that Mixed Content :

Mixed content occurs when initial HTML is loaded over a secure HTTPS connection, but other resources (such as images, videos, stylesheets, scripts) are loaded over an insecure HTTP connection[...]

Requesting subresources using the insecure HTTP protocol weakens the security of the entire page, as these requests are vulnerable to on-path attacks, where an attacker eavesdrops on a network connection and views or modifies the communication between two parties[...]

HTTP is not secure. Use HTTPS everywhere.

Solution 2:

Using HTTP instead of HTTPS exactly means the connection is not secure. One of the problems you'll face is that most browsers will give a warning if a securely-delivered page contains insecure elements.

The overhead of encryption is reduced by pipelining requests, which is possible with HTTPS but not HTTP, so from the end user's perspective you would typically not expect a large hit on the average web page.