Hi. I checked your sample shop with web speed test, GTMetrix.com, and the function ‘/?wc-ajax=get_refreshed_fragments’ response time is less than 300 ms while on our store is more than 2 sec.
This request is a part of the WooCommerce plugin and our theme doesn’t influence it. Could you please check how it works with some default WordPress theme like twentysixteen to understand is it our theme issue or not?
In general, this request is not very heavy and its response time depends on your server performance. It may be also influenced by some plugins installed but we can hardly advise you in this situation.