GTMetrix > Waterfall Chart Issue.

Yes, a redirect to HTTPS involves a 301 status. Avoid having any links the use HTTP and then no one will get the 301. As for existing search result links using HTTP, the 301 status should cause search engines to eventually update their links. This can take a fairly long time to happen.

As for the 404 image, you need to find the referring link in the page’s source HTML. Its location on the page will hopefully give you an idea about where it’s coming from in WP. Once you find its true source you can remove the reference. If the referring link cannot be found in source HTML, it was probably requested from JavaScript code.

Sorry but you didn’t find it 🙁 That image does exist. It’s probably the featured image of the page you found it on, the meta link placed there by an SEO or social plugin.

The problem file’s name is just 1.jpg, not Mobile-Coffee-Services-Singapore-1.jpg. Easy mistake to make. As I said, if it’s not in your HTML source, then it’s loaded by script. Finding it in script files can be difficult. It might be easiest to narrow down the plugin whose script is trying to load that file. You need an easier way to check for the request than gMetrix. Use your browser’s network developer tool.

You can use the network tool to learn the full URL of the problem file so you can check if it’s really there and verify it has proper permissions. The URL might even tell you which plugin it belongs to. Once you know the plugin, see if an update is available. Updates often solve odd problems. Sometimes updates introduce new problems. If you continue to have trouble with a specific plugin, I recommend contacting its dev through the plugin’s official support channel.