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Plugins, particularly SEO ones, take up huge resources of a WordPress site, so always better to be cautious. SEO by Yoast is a good plugin, but only if you really have a handle on what "search engine friendly" means so that you can set up and manage the tool properly. For Search Engine Friendly tools, suggest that website owners always start with signing up with Google Webmaster Tools first as Google has some excellent tools to test and make a Website search engine friendly. Once one has mastered Google's tools, only then I'd go for WordPress plugins.