Q: ATV4 slow to stream from iMAC?

When I browse to my iMAC to play a film from my ATV it takes forever to stream/start.  While, if I stream from my iMAC to the ATV it works straight away. 

Any idea where I can look to try and solve this problem?  It doesn't sound like a network issue but not ruling it out.  I am using an Airport TimeCapsule as my router so its Apple end-to-end.

Thanks.