Calculate the decoded size of the file from Bit Depth, Sample Rate, and Duration. It could be considerably larger than 81.4 MB.
If Roon can play no problem, is it decoding to memory?
When you are doing large transfers to your NAS, Windows is going to do memory management, and swap memory to disk. Even if you have 32 GB of memory, unfortunately. If MC only uses memory in a certain address range (say up to the 4 GB barrier), and Windows wants to use that address range for buffering of the file transfers, you are going to get swapping to disk. Hence, noticeable buffering of the audio.
Maybe there is a way to prevent swapping of specific application memory. Maybe MC could use memory at higher addresses to avoid this sort of clash. That's all more detail than I can get into.