Like start Firefox? I have no idea why it takes multiple seconds to bring up a blank window when starting a comparably useful program like Claws Mail is absolutely instantaneous.
To be honest, yes, generally programs that don't start instantaneously do something very unnecessary and stupid that has nothing to do with actual CPU performance. As in they read thousands of tiny files or they decompress files or they wait for some sort of answer from the network, or they make 10k+ expensive API calls or all of the above. Firefox is so big it might fall into the "all of the above" category, but to answer that question definitively you'd have to analyze what Firefox actually does.
And of course, making the decompression 15-20% faster by optimizing the decompression code (which is usually not even written by the developers of said software but just some external library) won't even make a difference because 20% less than 5 seconds is still 4 seconds which is way too long for a program to start. Instead using a different compression algorithm that increases file size by 25% but decompression speed by 10x would actually start solving the problem, with the next step being to ask why the program needs to read so much damn data at the start in the first place.
But since NVMe SSDs and Intel CPUs with very high boost clocks are quickly becoming the norm now even for laptops I don't see much of that happening, because Firefox starts pretty quickly (~1 second) on those machines.
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.
How big is the Claws Mail binary? A large part of the cost of starting up Firefox, or other large programs, is just getting the binary from disk into memory plus all the work the dynamic linker needs to do once it's there.
