Watch files is too slow

This is probably related to this issue
#220

However I found that running const newWatchFiles = typescript.getSourceFiles(rootFileNames, tsConfig) on every file change takes too long depending on the codebase.

I understand this is to watch/unwatch files to be compiled, however in the majority of cases the files to be watched don't change. If the watch/unwatch process can run in parallel, it would improve productivity significantly.