'hexo generate --watch' renders all files when one file changes, very slow

[jakeg]

I have a project with ~1,000 .md files. When running hexo generate --watch --debug, if I edit and save just one .md file I see:

18:08:18.210 DEBUG Processed: blog/abc.md
18:08:19.272 DEBUG Generator: page
18:08:19.272 DEBUG Generator: post
18:08:19.276 DEBUG Generator: asset
18:08:19.382 DEBUG Rendering page: 404.html
18:08:19.384 DEBUG Rendering page: blah.html
... [all ~1,000 files are rendered here]
18:08:31.882 INFO  Generated: blog/abc.md

... 13s elapses in which all ~1,000 files are rendered but only 1 is 'generated'. Is this correct? I would expect to save the one file and only that 1 file is rendered and generated, in well under 1s instead of 13s.

[stevenjoezhang]

See also #2164, #2519