Dicky is right here. There's a big difference between a steel cutting blade and a cutting disk.

The blade relies on having sharp serrated edges to cut. Get the steel blade too hot and it looses its temper and goes dull and soft and then fails to cut properly. So you want to work slowly and ideally lubricate the cutting area to remove heat.

The disk relies on a fast speed generating friction and heat, so it needs to run quickly. There's a trade-off between speed and the centripetal forces on the disk that are trying to pull it apart. If it's quite flimsy, then if you run it too fast, it may simply break apart without even touching anything. Add in the forces on one small section of the disk when it's brought into contact with the metal its cutting, and you can see that they can be quite fragile. The lighter the contact touch with the metal, the less likely the disk is to break. So whilst having the disk run as fast as possible is the ideal. For small, fairly weak, disks, you probably have to find a compromise between speed and the tendency for it to break.

Applying any sideways force on the disk once it's cut a groove and the disk will snap and break. It's easy to get a lot of leverage on the edge of the disk, so as Dicky says, it's important to keep the disk at a constant angle to the workpiece.