Magento is not page cache friendly
Page caching is the most powerful way to bypass Magento's heavy architecture and speed up slow pages, but even though 95% of the content on a Magento page may be the same for all visitors (and is thus safe to cache), items in a shopping cart or the list of last viewed items, for example, cannot be cached and shown to all visitors. Because of these small blocks that change per-user, traditional page caches cannot cache most Magento pages and are not able to speed Magento up significantly.
Hole Punching
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Hole Punching to the rescue
LiteMage Cache uses Edge Side Includes (ESI) to punch holes in pages where information changes from visitor to visitor. The remaining content is saved to cache. When the next person visits the same page, the cached content is served quickly, with only the holes needing to be filled in with data for that visitor. LiteMage Cache also caches per-user data in private caches, so entire pages, even those with multiple holes, can be assembled completely from cache.