Hello,

It is possible that something has not been installed/adjusted correctly, or it can be that some 3rd party plugin causes this issue.

Try to deactivate all of your plugins to make sure they are not causing this issue. If the issue goes away, then you can enable them one by one to find out which one is causing the issue. If you still have issues after you have deactivated all of your plugins, maybe you can try to disable Smooth Page transitions in Theme Options >> General >> Settings.

Also, please note that users can optimize site for speed, but good percentage is caused by server itself, so end users should increase performance on their own, and to to make sure that they are using reliable hosting service providers with high resources, and with fast and secure servers.

Here are some general tips for speeding up your website, and please note, you can't minimize files like js.php and css.php, because they are dynamically created.

1) Optimize your images with some tool: www.jpegmini.com/

2) Also you can install some plugin for cache, like WP Fastest Cache:

https://sr.wordpress.org/plugins/wp-fastest-cache/ http://wordpress.org/plugins/wp-super-cache/

https://wordpress.org/plugins/w3-total-cache/

or premium one http://wp-rocket.me/

3) You can enable gzip compression on your server. You should paste this code to .htaccess file located in your folder:

<ifModule mod_gzip.c>
mod_gzip_on Yes
mod_gzip_dechunk Yes
mod_gzip_item_include file .(html?|txt|css|js|php|pl)$
mod_gzip_item_include handler ^cgi-script$
mod_gzip_item_include mime ^text/.*
mod_gzip_item_include mime ^application/x-javascript.*
mod_gzip_item_exclude mime ^image/.*
mod_gzip_item_exclude rspheader ^Content-Encoding:.*gzip.*
</ifModule>

http://www.feedthebot.com/pagespeed/enable-compression.html

4) Leverage browser caching: http://www.feedthebot.com/pagespeed/leverage-browser-caching.html

5) Use at least PHP 5.6 or greater in order to process PHP code faster, and to fix older version bugs. Note that wordpress is now recommending PHP 7.0 or greater https://wordpress.org/about/requirements/

For any other issue you are facing please feel free to ask us and we will assist you as soon as possible. I will give my best to help you building your site.

Thanks & Regards, 
Vlada