Invalidate vs purge cache
Invalidating cache marks the content as "stale" and keeps serving the previously cached page. But the cache is updated in the background. As a result, your visitors will always receive optimized, cached pages.
Time to reflect changes: Upto 60 seconds
Making any changes in the Cache or Optimize settings will automatically invalidate the cache.
Purging the cache immediately deletes the cache from our servers. So the next visitors to your site will receive unoptimized, uncached pages, but fresh content.
Time to reflect changes: Upto 3 seconds