This article explains the performance overhead you may experience when rendering directly to a network directory, compared to a local drive.
Nuke renders files by using the POSIX APIs. This means that Nuke creates a .tmp file in the same directory as the final write location, and the data is added to this temporary file until the entire frame has been rendered. Once the render is done the .tmp file becomes the final output file.
Per the above process, Nuke is constantly sending small packets of data to the final write location directory, rather than creating a local .tmp version and copying it to the final destination at the end.
This has a positive impact on performance when rendering to a local drive, as all the packets reach their destination very quickly. However, this can create a performance overhead when writing to a slower directory, such as one on a network, making rendering over the network slower than rendering locally.