Low speed Wi-fi vs cable on with AirPort Time Capsule

Wi-Fi typically has lower bandwidth capacity versus Ethernet so some loss of performance would be expected. However, the amount you claim would be considered extreme.

Even though you have a "clear" path between the router and the iMac, it may be possible that there is significant Wi-Fi interference in the area that is preventing good bandwidth performance.

Try checking your iMac's AirPort connection to the Time Capsule first. Option-click on the AirPort icon in the OS X menu bar. Note the values for: PHY Mode, Channel, RSSI, Transmit Rate, & MCS Index.

For example, my mid-2011 Mac mini's values respectively are: 802.11n, 149 (5 GHz), -55, 450, & 23

• PHY Mode = The radio mode that your iMac is connecting with.
• Channel = The radio channel the iMac is connected to.
• RSSI = Received Signal Strength Indication. It's an indication of the power level of the signal as perceived by the antenna. Typically the higher the RSSI number the stronger the signal. RSSI is measured in arbitrary units. Some manufacturers use a positive scale, others like Apple, use a negative (0 to -100) scale. For the AirPort, the less negative the better.
• Transmit Rate = Instantaneous maximum data transfer rate of the connection in Mbps.
• MCS Index = Modulation and Coding Scheme. The MCS value identifies a number of things, including number of spatial streams used in the Wi-Fi connection between the iMac and the wireless router. A larger or smaller MCS value is meaningless. Instead it defines the actual connection type. In turn, the type of connection can play significantly on the potential bandwidth performance. In my example, my Mac mini had a MCS value of 23. That meant it had a connecton that used 3 spatial streams ... the most I could expect with my 4th generation AEBSn.

With a Transmit Rate of 300 Mbps, you know that your iMac's wireless is capable of transmitting / receiving data at that rate. To find out the actual data transfer rate, you would need additional tools to take the measurements.

Correct

Connect the iMac using an Ethernet cable to the Time Capsule, and you will have 1 Gigibit link speed.