Plugins that query large amounts of data and then perform an action with the data will impact performance. In general, plugins that perform overly complex operations should do them off server, and not in your WordPress hosting environment.
Good examples of this are the class of ‘Related Posts’ plugins that create FULLTEXT indexes on the “posts” table in MySQL. A FULLTEXT index is essentially a mechanism for making complex search queries against the content of posts, like “posts which contain A and B but not C or D.” It turns the MySQL database into a mini search engine, which really isn’t what MySQL was designed to be.
These queries become increasingly problematic as sites get larger and larger because the FULLTEXT index will continue to grow as well. At run time, the index will consume huge amounts of resources. Over time, the site slows to a crawl, and in extreme cases can crash the server if it runs out of threads.
Source : WPEngine