I have two PC coupled though 300Mbit Wi-Fi (WPA2 encryption) channel. I shared the whole partition on one PC and mounted it from other one through Wi-Fi.
I am able to reach 90Mbit/sec doing just copy and paste operation.
When I run MPC-HC (or for example VLC) to play 1080p (average bitrate is about 40Mbit/sec) movie though Wi-Fi, maximum transfer speed is about 40-42Mbit/sec (I use task manager to control this speed). And movie does not play flawlessly.
If I use KMPlayer transfer speed jumps to 60+ Mbit/sec and movie plays perfect.
It is obvious, there is nothing (almost) to do with SMB. Multiple components (like player, Wi-Fi link, file system driver, etc.) are involved in the playback process. Each of these components has their own “character” size of data they are optimized to operate with (cache size, buffer size, encryption/decryption size, Wi-Fi frame transfer size etc.). The entire system is not tuned up well.
So, I would not state that HD playback / WiFi issue is only determined by overhead/performance of SMB. Indeed, in wired ethernet environment, SMB and NFS perform pretty similar.
BTW, If you check MPC-HC developer’s site, you should find that ticket was open couple of years ago about MPC’s caching capabilities.