After a some research I finally found a way to fix that. If you're OK to use your MBP always connected to the AC Adapter, then that's the cheapest and easiest solution for you!
1- First download these two kext files:
2- Go to /System/Library/Extensions
3- Make a backup of AppleIntelCPUPowerMangement.kext and AppleIntelCPUPowerMangementClient.kext and then delete it from this folder
4- Add here the two kext files you've downloaded earlier
5- Open Terminal and do the following:
cd /System/Library/Extensions sudo chown -R root:wheel SleepEnabler.kext sudo chmod -R 755 SleepEnabler.kext sudo chown -R root:wheel NullCPUPowerManagement.kext sudo chmod -R 755 NullCPUPowerManagement.kext
6- Reboot and you're done!
If the computer doesn't boot, do this:
1. Hold down the option key while powering on
2. Choose [Recovery HD]
3. Open Terminal
4. Do the following:
cd /Volumes/[volumename]/System/Library/Extensions rm -rf SleepEnabler.kext rm -rf NullCPUPowerManagement.kext
6- Put AppleIntelCPUPowerMangement.kext and AppleIntelCPUPowerMangementClient.kext back in the /System/Library/Extensions folder
The fans are still on high, but the slowdown is totally gone. Is good to use a software like CoolBook to manage the energy resources from now on.