Flash meters do not use CdS cells, they use silicon cells (photodiodes), a different breed and, nowadays, fast to boot. CdS ones become obsolete by the mid 80's.

Photodiodes are used, for instance, in fiber optics communications where 'flash' time is of 1/1,000,000,000 of a second duration - or less..

I believe the fist camera manufacturer to use them was Fuji (silicon blue cells) by the late 70's.

Jorge O