[ALL THEMES] I Change Theme. Now, My Site Is Too Slow

For example, the old theme uses 150x150px images, but the new theme uses 160x160px images. In this case, because the system cannot find the correct image sizes, it uses larger sizes which are sometimes full-size (1920x1200px). The images are now really big which, as a result, slow down the website.

To fix this, after changing new theme, make sure you have “regenerated images”. We recommend using the Regenerate Thumbnails plugin. You just need to install and run “regenerated images” feature once. It will create new image sizes for your new theme.