WordPress being slow, a DNS problem

A friend of mine runs a WordPress site and recently moved to a different provider. The transition went smoothly and the site ran fine for three weeks. One day he made a post like he normally does and the site become very slow. He deleted the post but with no result. His site was still extremely slow, I mean it took almost a minute to load the index page. Installation of a caching plugin, deactivating all plugins, selecting the default theme did not help at all.
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