This custom module is uniquely problematic because it's content-versioned and contains LESS, which means it causes ResourceLoaderFileModule::compileLessFile() to be invoked when building version hashes for the startup module. Sometimes this LESS compilation is in cache, and then there's no performance impact, but when it's not in cache it slows down the startup module quite a bit. I profiled this startup module request yesterday where compileLessFile took 478ms (26.5% of the total).
This module is the only one that both uses content versioning and contains a LESS file, so it must be responsible for all of this time.
One obvious-seeming solution is to convert this module to use packageFiles, so that versioning will be done based on file hashes rather than the module contents. However, the module contains three script files, and all three are built using WebPack. So we'd have to either concatenate these three files into one file as part of the build process, or create a shim file that serves as the entry point and require()s the other files.