He doesn't specify what file system he's using, but offhand, you would assume that what actually takes time isn't to creating the file, but rather allocating all the blocks. A good first step would be to reproduce the issue and take a profile of both cases?

xfs, ext4 and btrfs all have delayed allocation. so they should only synchronously allocate the blocks if there is memory pressure or if they're triggering those well-meant (but in this case counterproductive) auto_da_alloc heuristics.