<< Does a 24X burn CD's exactly twice as fast as a 12X? >>
nope
This is from CDR-info for the Lite-On
Zone CLV. The data read/write rate keeps constant within a zone at the inner track then increases to higher constant rate at outer track as another zone. For example at 24X(Z-CLV) writing speed, the drive starts burning inner from 16X(CLV), switch to 20X(CLV) at 6min and switches to 24X(CLV) at 16min, this is so called Zone-CLV, 3 zone total.