No the content I watch is not HD or 4K, unless you consider 720p to be HD. I watch stuff in the 480p to 720p range mostly and do not have any trouble with it. Also some of the videos I watch are with the HEVC a.k.a. H.265 video codec instead of the older AVC a.k.a. H.264, but those videos are only 480p resolution (720x480 pixels like on an NTSC DVD).
So if your issue is occasional issues with 1080p content and severe lagging with 4K, that would probably happen to me too if I actually tried it, my Mac is not really built to handle anything above 720p though and the screen is 1280x800 pixels so I have no need for higher resolutions than my screen can display.
I did find an article on fixing slow SMB that occurs on Mac OS X 10.11.5 (the 5th El Capitan update) as well as later versions like Sierra and High Sierra. It might be helpful to you, apparently turning off signing of connections in SMB increases the data transfer speed to be several times faster:
https://dpron.com/os-x-10-11-5-slow-smb/
You should try that solution and see if it can increase your SMB transfer speed. Basically it will make things faster by simply sending and receiving the data as-is instead of encrypting it on the server end and decrypting it on the client end, since encryption and decryption is a CPU and memory-intensive process that slows things down and is completely unnecessary for shared video files on a LAN.
I tried it out myself and found that disabling encryption causes connections to SMB shares on Windows 10 servers from a macOS client to no longer work. So I undid the change. But if you CAN still connect, your SMB connection speed should be higher. It seems the SMB server must be set for encryption to be either optional or disabled, and in Windows 10 at least, apparently the default now is set to be always-on. From what I have read, there are ways to disable encryption/signing of SMB on both the client and server end so it has to be done on both ends. So I could get it working if I also disabled server-side SMB signing on the SMB server running Windows 10.