Actually timthumb is a little bit more resource heavy, but there is not much of a difference – at least I couldn't spot it. The non-timthumb version generates the images once, then loads them every time, so once an image is created it's used all the time. Timthumb does almost the same but after the page load, so the rendering is delayed, but microrequests are made towards the server to check the cache. Anyways, the results are almost identical.
But if you want to use timthumb anyways, then check the permissions on the plugins/related-posts-pro/includes/cache folder. The permission should be 777. Some servers however does not tolerate this level so you can also try 766 then 755 as well. If none of them works, then the problem is elsewhere, or the permissions are locked via SSH.
Same thing for the plugins/related-posts-pro/cache/ (folder only) and plugins/related-posts-pro/css/ (folder and all of the contents!). After changing permissions on the css folder the error message should disappear after saving the settings regarding the “Error saving stylesheet…”.
If the error does not disappear then you can still try to turn off the inline styles option, it may still work. (there is a tiny chance the the server reports different permission values to the script than it really is, and it triggers the message)Best,
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