You have to wait for ~10 – 15 sec if you watch a TV recording, e.g. from a MediaPortal server in your network, the channel’s resolution is 1080p and you press fast-forward or rewind. It works without problems with 720p or lower resolutions.
You might see a similar behaviour watching a 1080p movie LAN.
Having a look at the xbmc.log file at the client, you see hundreds of error messages like
DEBUG: AddOnLog: MediaPortal PVR Client: SeekRec: iWhence 0 pos -1597861956
during the waiting time.
Update the drivers of your server network card and the problem should be gone.