Q: Very very slow internet on MacBooks

I have 4 ideas (most likely none of them is the solution, but they popped into my head, and needed to get out )

Check that each Mac has a unique System Preferences -> Sharing -> Computer Name.

Check that each Mac has a unique IP address via System Preferences -> Network -> Advanced -> TCP/IP

Put your Macs into IPv6 "Link-local only" via System Preferences -> Network -> Advanced -> TCP/IP -> Configure IPv6: "Link-local only"

Make sure you have good working DNS server IP addresses via System Preferences -> Network -> Advanced -> DNS.  You can experiment using some well known DNS servers such as

OpenDNS.org

208.67.222.222

208.67.220.220

Google DNS

8.8.8.8

8.8.4.4

You would use the [+] button in the DNS display to add your own DNS servers.  You can later use the [-] button to remove them.  NOTE:  If providing your own DNS servers actually works, then you need to look at your router (typically the local DNS server), and the DNS servers provided by your ISP.

Again, chances are this is not your problem, but if no one else provides a better guess, it is something to try that is non-destructive.

If your Wi-Fi source is capable of generating a 5 GHz Wi-Fi network, try that to help avoid conflicts with Bluetooth.

Troubleshooting Wi-Fi issues in OS X

Wireless Connection Problems - Fix

Wireless Connection Problems - Fix (2)

Wireless Connection Problems - Fix (3)

Wireless Connection Problems - Fix (4)

Also try turning off Bluetooth.

Slow Wi-Fi - Mavericks

You are welcome.