How fast a LAN Network works is based on the SLOWEST component. Therefore if you have a workstation with a 10/100 NIC hooked up to a GLAN Hub and additional workstation with a GLAN NIC you will get 10/100 while the slow workstation is active and 1000 when the slow workstation is turned off.
Your router/hub is capable of highspeed, but one or more computer isn't. You can sometimes fix this by swaping the slow NIC out. If the slow workstation is a laptop/handheld, it's often not worth it. Just turn off/disconnect the slow workstation when it's not in use.
Your "fast internet" isn't capable of even 10/100 speeds (often 8kps), this is the one device that doesn't slow down the other devices. So, if you are measuring your network performance you have a slow device/workstation, if you are measuring internet performance you have a fast (moderately) connection.