Here’s Vishay’s app note: Faster switching from standard couplers (PDF!):
Applications employing lower-performance “standard couplers” can still be optimized for switching speed. Thoughit can not be expected that these devices will perform to the level of specialized “high-speed” devices, they can be more than adequate for many switching applications, up to the 1 μs range. These standard couplers are the subject of discussion in this document. They fall into two general categories, 4 pin couplers, 6 pin couplers, as well as a special configuration of the single transistor coupler.