jump right and left only at a fast speed?

It's hard to answer without any other information, but if you're using the platform behavior you'll just use a simulate control actions for jumping when each of those buttons are pressed. And another for moving left or right when the character is not on ground.

The problem is the platform behavior is not super precise. Some deviantion may happen. In this example, you sometimes get the character a little off to the left or rigt. I wasn't able to isolate a reason for that, though.

See Example

Use A to jump left and D to jump right