Current Transfer Ratio
The specification you are looking for is called the Current Transfer Ratio, or CTR. A typical CTR value for a 4N25 type opto is 30% to 600%. You can specify a CTR range when you buy them and the higher the CTR the more expensive. They won't quite get to a factor of 10 and they degrade with age so you really can't depend on the CTR remaining constant.
When you say they are slow I assume you mean propagation delay. They are in the 3 to 5 microsecond range. If you want low propagation delay try the HCPL 0710 at 40 nanoseconds. Of course you will pay for this feature.