It is a known problem, your site is slow and the consequences are there. What happens when a site is too slow? Well, the reverse from above, so it's bad for your ecommerce performance. On hosted platforms, this problem is less common than when you have an open source or self-built shop. There is also a difference between mobile usage of your site and just (desktop) visit. In general, all the sluggish elements are even more slow on mobile. The cause of slow webshops can be diverse, but some things to look at:
Test your website (or the shop you want to purchase) with the Google Pagespeed Tool. This tool gives a grade for the speed of your site. For many improvement points, a developer needs to update your site to improve it. There are also known or common problems (such as slow Magento webshop), which are sometimes difficult to solve or where you really need the right expert, sometimes at substatial cost.
When buying an existing web store, one also takes over the content, server & software. A good (fast) web store has more value than a slow one. If you have technical knowledge or expertise, there are opportunities here as well. For a seller, the reverse applies: Increase the value of your site by creating a shop that performs well.
If you take over a (slow) ecommerce site then you can also choose to migrate to a better platform. Old open source or self-built webshops are difficult to improve and often expensive in maintenance. In such a case, there is a need for content migration and continue the shop on a new site.