Hello,
1) I have tested your shop page speed with the default Storefront theme and with the XStore theme and speed results are almost the same. Check and compare:
Storefront – Fully Loaded Time 7.4s https://gtmetrix.com/reports/atsalis-jewellery.gr/ZNsYjTIm/
XStore – Fully Loaded Time 7.6s https://gtmetrix.com/reports/atsalis-jewellery.gr/VN9RvWuT/
I would recommend you to contact your hosting provider and ask if they could improve the site loading from their side (reduce server response time etc)
2) You can add widgets to the filters are at the top of the products from Appearance > Widgets > Shop filters https://prnt.sc/1w8mjjs
To find settings for that area go to Theme Options > WooCommerce > Shop > Shop filters https://prnt.sc/1w8mn36
3) Read here how to speed up the Revolution slider loading time https://wpmudev.com/blog/optimize-slider-revolution-wordpress/
Regards