I had a similar problem, when installing XP (to replace W98) on an old 300MHz Toshiba Satellite. Every tool (including XP) reported the CPU to be 150MHz.
After a few days they all then reported 300MHz - without any action on my part. The doubling of CPU speed made no difference to the performance, it was still very slow compared to W98.
I've assumed the latest report to be correct but can't explain the reports immediately after installing XP.