How do I fix a slow toilet flush?
From a similar question's answer


  If the toilet still drains slowly, then you have a blockage that may
  not be solved with a new toilet. It could be an improperly installed
  wax seal, perhaps the drain line isn't properly sized, or there's
  something that can be snaked out. It's also possible you could have a
  venting issue, but I'd expect bubbles, gurgling, or bad smells with
  your other plumbing if that were the case.