Plain-text formats have always been slower for things that are not plain text. But even 30 years ago, when computers were even slower, Unix designers decided plain text was still the way to go, because it was easier to debug and easier for humans to work with. No specialized tools required, no poring over hex dumps. HTML won over other document formats. JSON and XML won over other binary formats. Any coder can look at JSON and see what is being transferred, without the aid of anything but a text editor. Plain-text marshalling formats for binary data (e.g., base64) are still useful for pasting data into an email or adding ssh keys to authorized_keys with "cat >>".
This is perhaps a long way of saying that I don't expect that having tool support makes SPDY any nicer.
