Example of optimizing conditional filters using a Chunk wrapper

optimized.html

[[[[*page-image:notempty=`$wrapImgThumb? 
  &src=`*page-image` 
  &alt=`[[*pagetitle]]` 
  &thumbOpts=`w=480&h=320`
`]]]]

original.html

[[*page-image:notempty=`
  <img 
    src="[[*page-image:phpthumbof=`w=480h=320`]]" 
    alt="[[*pagetitle]]" 
  />
`]]

wrapImgThumb.chunk.html

<img src="[[[[+src]]:phpthumbof=`[[+thumbOpts]]`]]" alt="[[+alt]]" />

Technique proposed by Jason Coward

Use this technique to prevent MODX from parsing all tags in a conditional.
For further understanding read this: http://modx.com/blog/2012/09/14/tags-as-the-result-or-how-conditionals-are-like-mosquitoes/