Q: Very slow internet

When the network is slow, 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

RSSI

Transmit Rate

MBP slow on ethernet also?

Try the basics first:

   Change your router channel number.  Most times this works & is all you have to do.

Disconnect & reconnect your modem.  unplug it for about 10 seconds.  Plug it back in.  Do the same for Apple’s routers.  Wait for everything to reboot.

System Preferences>Network

Click the Assist Me button.

In the next window that pops up, click the Diagnostic button & do the necessary.

Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer.   See these basic networking KB Articles:  http://support.apple.com/kb/HT1401 AirPort troubleshooting guide

http://support.apple.com/kb/HT2712 Using network locations in Mac OS X

Manually provided DNS server addresses are higher priority than DHCP's

http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting

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What to do when you can't connect to the Internet

Also, run the Airport Utility app which is located inside the Utilities folder.

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If using a  Linksys router, contact LinkSys Customer Support and/or post in their forums.

If using Apple's Airport, please re-post over in one of the AirPort Forums.

That looks OK. The only theory that fits your description of the problem is that you're being affected by interference that's only present at certain times of day. All I can suggest is that you read this:

AirPort and Bluetooth: Potential sources of interference