Evidence says there's not a significant difference between the speed of recognition for 'line-based' vs. 'filled' icons.
When it comes to icon readability (interpreting 'readability' as inverse to 'time it takes to understand') other factors are more important than style differences.
Here's an excellent, annotated article from boxesandarrows.com covering how to optimize icons for faster recognition, which covers an actual experiment done on lined-vs-filled icons. It's also got great references that include published papers in visual theory, semiotics and linguistics.
There was no noticeable difference in speed of recognition for the two
styles. So we could suggest that, if an icon is well designed and
represents a particular recognizable shape clearly, its style doesn’t
have a significant effect on the speed of recognition. The more
notable difference, however, was in how individual signs relate to the
concepts they represent–the relation of signifier to signified in
The icons that performed faster in both sets were
iconic signs–‘calculator,’ ‘camera,’ ‘mail.’ On the other hand, the
icons that seemed to have required a short pause were symbolic:
‘download,’ ‘back,’ ‘copy.’ Even the ‘delete’ icon caused a slight
delay in participants’ response.
Hope that helps!