Thanks Anna. yea, I understand that WP-based website may not be quite suitable for really large sites that contain over some hundred thousands of products and it would definitely need customization to handle that many products or completely move on to different platform such as Magento. but, experiencing performance issue with just a little over 3k products is something I would’ve never expected..
Kleo sounds interesting but I think I like re:vendor better just becuase it doesn’t need much customization and has many features I’d require. I am checking a few things with the theme author and had my engineer look into the theme as well. I will let you know how it goes.