Believe me I tried a number of things. It was like that from the first run and every time.

Well, technically it might have used 700ms the first time and 400ms later, I didn't measure the first run, but it was still way faster than on Windows which was why I started measuring using time after all.

Also on Windows it kept being slow on subsequent runs, and in my world that matters too.

I don't have recordings, but I recommend you try for yourself. I compiled both from the same source, and as far as I can remember I used standalone compilation for both (I don't have perfect memory but I have a history of being careful.)

Spawning a process is a rather heavyweight operation on Windows, compared to Linux, so it's not surprising that it's slower. But 5 seconds definitely makes it sound like an I/O caching issue.

Another common problem on Windows seems to be antivirus. Many of the available options seems to insist on checking every file on every invocation even if I have compiled it from my own sources (I see this can be hard to judge) and even if it last checked it a few seconds ago.