On a mathematical approach:
15 seconds for a file of 245MB means: 16.3MB/s
This equals 130.6 Mbit/s
This is already more than a 100Mb NIC can transfer. I agree for a Gb network this could be improved but it can be any part of the chain between the sending device and the receiving device that acts as the bottleneck of your network read/write performance. If you want to know what is your bottleneck, you have to identify every single item in your network involving network performance and test it.