Loading a View Through Ajax - Slideshow not showing
Are you using a Drupal module like Views Slideshow to create your slideshow? 

Usually slideshows will need to dynamically calculate the height/width of the content in the slides in order to render correctly. If the slideshow is hidden when the slideshow's javascript is loaded, then there will effectively be no content for the slideshow's javascript to render, thus the slideshow will never render correctly.

Does the slideshow render with no problems if you remove your CSS and/or JS that hides (and then slides down) the slideshow?

I haven't dealt with this sort of issue directly, but hopefully I can point you in the right direction. I would guess that somewhere you are using JS or CSS to hide the #portslide-wrap div (i.e. display:  none;) until it is ready to slide down? I would suggest trying to hide that div behind some other element using z-index rather than display:none(so that the slideshow can render while being obscured) and then hide it using JS after the slideshow has processed (probably a class like "slideshow-process" is added to a wrapping div, which you can use to hide the slideshow via CSS).