Onur, please clarify what the "website size" you mean.  If you are referring to the index page, so please clarify what tools you used to test it. And you mentioned other sites which are faster then yours, please could you give some examples. Do you have some Oxwall sites and one of them is loading slower than others? 
 
 
Concerning the loading speed of pages it depends on different factors, for example:
 
- heavy graphics elements
- a large number of components, widgets 
- server capacity 
 
And it is important to know: 
- how many user do you have online?
- what hosting plan do you use?
- if you use shared hosting account you may have such speed problems due to server overload cos you can't control server at all.
 
So to solve your issue you should take into consideration all these factors.
 
Also make sure that you have dev_mode and debug_mode disabled in ow_includes/config.php file  (https://docs.oxwall.org/plugin-tuts:enable-debug).