I asked this question on Khronos KTX forum.Here is the answer I got from the forum moderator:
I have been told by the NVIDIA OpenGL driver team that the Quadro 4000 does not support ETC in hardware while it does support DXTC. This means the ETC-compressed images will be decompressed by the OpenGL driver in software then loaded into GPU memory while the DXTC-compressed images will simply be loaded into GPU memory. I believe that is the source of the performance difference you are observing.
So it seems like my card's hardware doesn't support ETC.