In case, your MyBB ACP -> Configuration -> Server and Optimization Options -> Enable search engine friendly URLs? is set to DISABLED then use the following robots.txt.
Any image when uploaded into wordpress, it will automatically creates few more instances of that image of different size based on media settings. Recommended media settings is shown below:
You should create robots.txt in the root of your wordpress installation if you don’t have this file. Typical Example of Wordpress robots.txt is shown.
From the GtMetrix and Google Page Speed result, you can notice that the jquery is loaded at the time of site loading resulting increased initial loading time.