Keito said:
If you can tell me how to implement API, i'd do that.

I don't personally tag my local files, but I figured I'd chime in on this.

If you can find yourself a JSON parser or XML parser to replace your HTML parser, using the API is quite simple. Just replace your request URL https://e621.net/post/index/1/md5: with https://e621.net/post/tags.json?md5= or https://e621.net/post/tags.xml?md5= and make whatever small changes are needed to tell the parser to give you just the tag names from that.

Here's an example of the output:
https://e621.net/post/tags.json?md5=466A53E868164275F9386F0A90A3FE8F
https://e621.net/post/tags.xml?md5=466A53E868164275F9386F0A90A3FE8F

JSON is slightly more efficient than XML, but either should give your program a significant speed boost.

Here's a link to the full API documentation if you wish to learn more: e621:api

Cheers. :)

edit: whoops, parasprite beat me to it.