To fix the slow sync problem in the ethereum miner, you need to:

  1. backup your account(s) and application data (from the eth wallet if you're using it)

  2. remove the chaindata content via

    1. delete the directory (easiest/quickest way)
    2. use the geth removedb command to do it properly

  3. reload the database via:

    geth --syncmode "fast" --cache 1024 console

Now, if your database is not in the default path, (e.g. not in %APPDATA%\Ethereum on Windows), you'll need to tell geth where it is. You do this with the --datadir path/to/data/directory. So, my command looks like:

geth --datadir "D:\Coins\Ethereum"  \
  --syncmode "fast" \
  --cache 1024 console

Bonus

To monitor the current state of affairs, you can use:

    # Connect to geth and get its prompt
    geth attach

    # Show current state
    > eth.syncing

HTH,