[bzbarsky]

Fwiw, Firefox stores almost everything it will need on startup in one large file (omni.ja) precisely to avoid the "thousands of tiny files" problem. That data is uncompressed, precisely to avoid the decompression problem. The low-hanging fruit has largely been picked.

As for why so much data needs to be read... I just checked, and on Mac the main Firefox library (the executable itself is mostly a stub) is 120MB. So that's going to take a second or three just to read in at typical HDD speeds (faster on a good SSD), and then the dynamic linker has to do its thing on that big library, which is not instantaneous either.