It seems to be more like instant>instant and slide>instant.
1. The a.alt-toggle element isn't actually being animated, its display property is being toggled between "none" and "inline-block".
2. By calling .hide() on .alt-content on line 15 in your jsfiddle, you set its display property to "none". When slideDown gets called on the element, lines 308-312 in the effects module gets skipped which is why it shows instantly. If you inspect the element though, you'll see that the height property is being incremented.
When you try animating inline elements, you're gonna have a bad time. Height and width properties do not apply on inline elements. Your best bet is to use a block-level element for your .alt-content like a <p> or <div>.
There's probably a dupe of this somewhere in the tracker (and likely closed as notabug).