Is turbo lag caused by low speed or rpm?

The turbo charger is driven by excess fuel that is burned on the hot (exhaust) side of the turbo. The more unburned fuel the faster is will turn till it gets to its max. RPM and the boost also goes up and the RPM of the turbo compresses air on the cold side of the turbo. On a steady pull on a hill you will see constant boost and when on a level road the boost will drop because the engine is burning more of the fuel going into the cylinder that going into the turbo.