Hello crisbeto and welcome to Seven Forums.
This is one of those problems that might take some heavy duty troubleshooting since it doesn't always happen. Some of the things that could affect internet connectivity could be the antivirus suite you're using, scheduled tasks running in the background like defrag, malware on the laptop, etc. Have you tried booting into Safe Mode with Networking to see if there's any difference in connection speed?
Advanced Boot Options
You may also want to go through these tutorials to see if you can isolate your problem.
Troubleshoot Application Conflicts by Performing a Clean Startup
Troubleshooting Steps for Windows 7
Another common problem is an outdated network driver. This Microsoft article goes into more detail.
Slow internet connection Windows 7 PC - Microsoft Answers
And some more general information regarding internet connectivity:
Why is my Internet connection so slow?