Hi, I have tested your website. It is getting slow not only because of invalid javascript and css, It has some more issues such as there are lot of resource files(images, media, css etc) which does not exist and being called by your webpages and getting connection problem. Thus producing very slow result.
Other than this, There is no server caching, client caching, js and css aggregation are implemented.
Thus, We will do following to increase the overall experience:-
1. clean javascript and css files.
2. clean and remove files non existing files linked by pages.
3. implement server caching,
4. implement client caching,
5. implement js and css aggregation as much possible.