1, When possible, avoid running back up utilities during peak hours of your site’s operation. For most ecommerce sites, it is best practice to reserve a time after peak hours (typically anytime between 12am-5am). Additionally, avoid running security scans, such as McAfee Secure, during peak operating hours. This will ensure your customers are not leaving your site due to excessive latency.
2, Avoid using too many different external sources because every DNS lookup takes extra time and could cause latency with your site. Having images, iframes, twitter feeds, etc.. pulled outside your server will slow down your site and page load time. When possible keep what you can on you own server as this will speed up your site tremendously.
3, Make your output W3C compliant. W3C errors can slow down the browsers. Only have extensions to other technology providers that you actually use. Any extensions that you are not using can put a strain on your servers.
4, When possible, limit the number of products on a product overview page. This will help your pages to load faster as there are less images and information that the browser will have to pull in.
5, Use Magento’s Compilation feature. It’s reported to give you a 25%-50% boost in performance. This can be accessed via: System > Tools > Compilation.
6, Enable full page caching. This will help product load faster on your ecommerce site.
7, Enable Solr search, usually this is faster than the default setup. This is more noticeable when you have lots of products, more than 10 thousand for example.
http://www.slideshare.net/dmitriysoroka/methods-and-best-practices-for-high-performance-ecommerce
Slide 32 has a chart of this Slides Created by Varien.
8, If you are having planning a big sale or advertising on sites such as slickdeals, dealdump, techdeals, etc, contact your support team so you have time to prepare for the large influx of traffic to your site. This will allow options for increase bandwidth, additional failover options and so on.
9, Last but not least monitor your site. You need some sort of monitoring in place in order to see problems before they start. It is recommend to have dedicated hosting with monitoring support. When a possible problem is discovered early it is often times preventable.
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