preprocess is awfully slow

Files affected

Which file(s) is(are) affected

• preprocess.src

Steps to reproduce

A step-by-step explanation of the way to follow and the configuration to have to reproduce the bug

• Build a source file with 1 or 2 libraries

Expected behaviour

The file to be built under a dozen of seconds

Actual behaviour

The tool is writing each line of the source code at a time, at a pace of a few lines per seconds

Possible fixes

My own preprocessor became faster, so it's not based on the idea of bundling source files together
A likely cause could be that the tool writes directly the content to the file, rather than writing everything at the end
If GH added a small delay over disk usage, it would drastically reduce the performance of the preprocessor