Hi Vladislav,
As this is just a static HTML template, there is the main reason why the equivalent application using HTML/template is slower than PHP variant.
HTML/template rendering code heavily uses reflection and methods with a variable number of arguments and they are just not as fast as the statically compiled code.
Moreover, please learn more from here https://gtmetrix.com/why-is-my-page-slow.html
Furthermore, if you need the WordPress Theme version of Calldee then you could contact our sales team to get a quote.
Best Regards,
ThemeLooks Support Team