I'd say it almost halves my reading speed. In regards to speed reading I've always been under the impression it means you take in entire lines/sentences in one go rather than reading faster, skipping the sub-vocalisation step. For instance the homepage of HN is a quick scan vertically down the page until a headline catches my eye as interesting, no real side to side eye movement needed unless the title is a bit long

Maybe this brain reading faster than eyes thing is best aimed at sub-vocalisers? I'd be interested in knowing if the people this helps read 'out loud' in their heads