Hi amnessiacc
I just tried to replicate at my local by installing S5 Masonry module to GCK Store template but it loads pretty quickly.
You can try this way:
1 - Disable all 3rd party extensions temporarily on your site, I see you're using so many of them
2 - Backup your site and install latest version of template and S5 Masonry module, you're using old versions now
I tried to disable the same but your site blocks access from my place.
The webmaster has forbidden your access to this site