When the file is placed on a network drive, then the flush request is converted to a SMB_COM_FLUSH message sent to the server, which causes the server to flush its cache and only when it is done, then it responds back to the client. Instead of just affecting the client machine, then it also affects the server, which can cause very slow performance for all clients using the server.
It is possible to configure the server so it will not perform the commit operation, and thus avoiding the slow file operation (The client will have flushed its own cache):
Related Configure hard disk cache to ignore flush requests
[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \LanmanServer \Parameters]
TreatHostAsStableStorage = 1 (Default = 0)
Note this should only be considered if the server is equipped with a redundant disc system along with battery backup.
More Info MS KB840390
More Info MS KB894372