The HP Slate 500 uses a Broadcom WiFi card, device ID: PCI\VEN_14E4&DEV_4727&REV_01. When AC plugged, this WiFi uses 12-15% CPU constantly, which makes the Slate hot and slow. While powered by battery, no problem.
The fix is to download and update to this driver:
Note: don't click on the big "Download" button, the real download link is at the right bottom "Save file to your PC: Download".
Unzip the downloaded Wireless_Broadcom_220.127.116.11_Win7x86x64.zip somewhere, then go to Control Panel, Device Manager, find the Broadcom 802.11n under Network Adapters. Right click on it and select "Update driver software...". Then select "Browse my computer for driver software", and browse to the folder where you just unzipped the zip file to. It takes a while for win7 to verify the digital signature, when it finally prompts, click "yes to install it anyway". It will take another while, be patient.
You may verify that now the driver version is 18.104.22.168 to ensure a successful update. Now plug in AC, connect WiFi, press the alt-ctrl-del button, select Start Task Manager, you should see CPU usage is now down from 12-15% to 0-1%.
If update was not successful, you need to uninstall the old driver first. Be sure to check "delete driver file", otherwise win7 will reintall automatically the old driver when you rescan for new devices. Then manually install the new driver as described above.