Posting topic seems very slow

Today I posted a topic to a forum with 300+ subscribers. The spinning wheel took a very long time so I did hit the Submit button again, leading to 2 topics and 2 emails to each subscriber.

Is the time it takes to submit a post dependent on the number of subscribers to the forum?

In another test I noticed that the popup for writing the topic does not close when you hit the submit topic button. The spinning wheel stops after a while and the popup is still open. Clearly user don't know the topic has been submitted and hit the submit topic button again. Is this expected behavior?