wi-fi on iMac

Your description indicates a DNS issue. I recommend that you stop using your ISP's default DNS settings and begin using a third party's such as Google.

How do I change DNS on Mac?

1. System Preferences
2. Network
3. Click Advanced. 
4. Click the DNS tab
5. Click the little + sign at the lower left to add a new DNS server
6. Type in the numbers of a public DNS server (see our suggestions in the Windows section above)
7. Click OK
8. Click Apply

8.8.8.8 or 8.8.4.4 = GoogleDNS

Which band are you using to connect to your wireless router (A/B/G/N/AC/AX) ?

Is your router handling other traffic that may interfere with your iMac's WLAN card?

If you are connecting using 2.4GHz - are you using any other 2.4GHz devices such as bluetooth ?

Microwave ovens have the ability to disrupt WiFi.

Check DNS in Network - you may have to point to a different DNS server.

Please restart in Safe Mode and then retest.

Use Safe Mode to isolate issues with your Mac

Your welcome. Please click the solved button so other users may benefit.