One of the reasons to keep single-file SVG icons was that it tested
our renderer. Now that we're switching to Cairo anyway, this reason is
gone, so indeed we could switch to something faster.
In general, if that was the main reason, shouldn't something like that be put into a test suite somewhere, so the user isn't forced to wait for a "test" to run every time the program starts up? How many other "tests" are slowing down Inkscape in various parts of the code? Maybe I don't understand the intent of what you're saying.
It would be very nice to see quicker startup times!
- Bryan