Q: my iMac wireless connection slower than macbook pro?

Please check the speed of your connection on this site.

Then start up in Recovery mode. In the OS X Utilities screen, select Get Help Online. A clean copy of Safari will launch. No plugins, such as Flash, will be available. Check again. Then restart as usual and post the results.

If you connect to your router via Wi-Fi, please connect with Ethernet and see whether there's any difference.

I'm not sure I understand your response. Are you saying that Ethernet is much faster than Wi-Fi?

Hold down the option key and select the Wi-Fi menu in the menu bar. What values are shown for the following?

          PHY Mode

          Channel

          Security

          Signal/Noise or RSSI

          Transmit (or Tx) Rate

Please do not post the BSSID, as that information is private.

Now hold down the option key again and select

           ▹ System Information... ▹ Network ▹ Wi-Fi

Compare the two-letter Country Code with the ISO code for your country. Does it match? The code X0 means that the country couldn't be determined. The code X3 means South America, not otherwise specified.

USB 3 devices can interfere with Wi-Fi and Bluetooth. You may be able to mitigate the problem by using a USB cable with better shielding, or by switching from the 2.4 GHz wireless band to the 5 GHz band.