You shouldn't really have speed issues if you are using a network fileshare on a LAN (eg. not going over a WAN link). Latency is important as is bandwidth for large files. Are you using a gigabit ethernet connection? Have you tested regular file transfer speeds (eg. copying a file from the network to your local machine)? With gigabit you should be able to achieve close to 100MB/sec transfer speeds, eg. I can get 80MB/sec on my home gigabit LAN. If you are only seeing a few MB/sec then you may have a problem with your network configuration or the network server.
The other thing that comes to mind is if you are opening your file via \\server\share or if you are mapping your network filesystem to a local drive letter (eg. Z:\share). Eg. you may find Z:\share faster than \\server\share.
Failing my advice you could open a support ticket with PTC or investigate your network performance using more detailed tools such as Wireshark.
