glTexSubImage2D fighting glCopyTexSubImage2D

I have a strange problem with nVidia driver/hardware. The hardware is GeForce2/3 and the driver is 23.13 (on Linux)

I want to glTexSubImage2D into level 0 of a mipmap texture, and glCopyTexSubImage2D into other levels. Each by itself works very fast. But when I do both, glTexSubImage2D becomes terribly slow - like hundreds of milliseconds.

Are two different internal texture memory representations lurking in there??