A wp site well optimized and using the minimum simplest theme. Still in page speed of Google its scoring less than 60. I don't understand why?
More or less every theme produced by wp is heavy loading these days. I don't know from where you have gotten a simple looking theme but if its simplest then it maybe also not mobile optimized fully. Have you checked that?
And I am assuming that you are not using any unnecessary plugins and what nots. So, check the page-speed insight itself. It itself shows a detailed analysis on what factors your page is failing.
According to search engine website loading speed should be more than 90. If your website speed less than 90 than it does not follow search engine guidelines.
Check for the areas where you can improvise, as suggested by the tool!
Seems like most of the issues are with CSS & JS.