Max: 144Mbps.
Quite okay leh for gigabit network. Maybe caching on player side got issue therefore stutter.
I know my Z9X sometimes got stutter playing the 200Mbps Jellyfish test file over network/SMB. 400Mbps Jellyfish confirm will stutter on my Z9X. But then 400Mbps si bei extreme, so not counted lah.
For NAS, it also depending if there is any prior arranged CRON jobs running in the background, or any copying, download, extracting on going before you start watching a movie. This will affect and potentially cause the stuttering when playing the movie
So far I don’t find any stuttering when I movie as I make sure there isn’t any ongoing tasks in my NAS
A tip, assign your downloads to a single drive volume separately from your movie folder. Ongoing tasks will not affect the streaming of movie from NAS to player since the drive are being read separately and the writing to the drive will not affect the one being read. 
Since we are on this topic of streaming high bitrate video files from NAS, can I check what is your physical connection between the NAS and the media player?
I’m not using a direct UTP cable from my NAS to the media player (Zidoo). Instead I’m using a powerline connection which is less than ideal.
My connection is as follows:
AV2 Gigabit Powerline (Host) at my living room which is dedicated solely for my media player > Router > AV2 Gigabit Powerline (Slave) in my home theater room > Media player (Zidoo UHD3000).
As you can see, I am NOT even using a direct Ethernet cable connection between my NAS and the Zidoo. This is why I will need to implement a separate HDD docking station for playing of high bitrate video files like true uncompressed 4K UHD titles.
Even with this subpar connection, I can still stream normal MKV, BD ISO/BDMV and 4K UHD (compressed) video files without stuttering except for the above mentioned scenario.
That is very true. Make sure the NAS ain’t doing any BT downloading in the background. You may want to stop the task and resume after watching the movie.
Next, a good router with proper QoS setting can make a difference to streaming as well. Make sure to set media streaming as high priority over other tasks like web surfing, downloading or gaming etc. it does make a difference.
Router>Switch>NAS>Oppo
Using NFS protocol
Switch QoS priority over others. Movie first, music 2nd. But priority will auto switch between movie and music, followed by rest of streaming. Either way you will only do either one than the other hence priority is always there. So far no stuttering even playing iso files. 1Gbps LAN throughout the chain for sure for my case
Also you will want to make sure that you have an unmanaged network switch. Some network switch also have built-in QoS on certain Ports (maybe 5 & 6) for priority connection etc. These are some of the things to look out for.