Fresh new install or keep old version

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There’s nothing about moving a site and/or “purging” and re-installing a large PHP application that will “clean” anything or have any impact on performance. The things that matter regarding performance when going to a new host:

  1. Database settings and tuning.
  2. PHP settings, especially if you can to to PHP 7 and run PHP-FPM with proper pool setup (if on a multi-core host plan with dedicated core resources).
  3. Going to SSD storage instead of spinning disks.
  4. In general, memory and CPU – do you have enough of both?

A good hosting company will work with you on all of these points.