Based on the numerous e-commerce web sites I see on a daily basis, it's usually one or more of the following: * Inefficient, disabled, or non-existing caching.

Based on the numerous e-commerce web sites I see on a daily basis, it's usually one or more of the following: * Inefficient, disabled, or non-existing caching. * Random SEO plug-ins/add-ons doing things like queries on non-indexed data or insanely convoluted JOINs. * Logging every single HTTP request into a single table that nobody ever remembers to truncate. * Multiple queries for each page in order to recommend similar products in the page footer or on some widget. * Checking whether the visitor is logged in, and if there's anything in the basket yet. * Checking whether there's an active discount for the current user/product combination. It's really quite ridiculous. The worst offenders are WordPress/WooCommerce sites. Dedicated e-commerce solutions usually fare better.