This web page lists Intel Core2 Duo CPUs for Socket P and the T6600 is almost at the top of the performance range for that series.
https://www.cpu-world.com/benchmarks/Intel/Core_2_Duo_T6600_single.html
Are you trying to replace a dead CPU? If not, there seems little point in looking for anything faster. Only the T7500 is shown as fractionally quicker and I've no idea if the HP BIOS will accept different CPUs. I'd recommend contacting HP to see if they locked the BIOS to work with a restricted range of CPUs.
The T6600 was released in Q1 2009, so that probably makes your laptop design around 14 years old. It should be fine running Windows XP or Windows 7, but it will struggle with Windows 10. Alternatively you could install a Linux distro specifically designed to work on old hardware.