I have my photo library in two places Aperture and Lightroom. The images in Aperture are small thumbnails, designed to function as a contact sheet easily searchable with keywords (thousands of photos in projects). The Lightroom version are full sized without keywords (and with fewer images per folder). The file names are the same. How do I incorporate the keywords into Lightroom (and maintain quick searchability)?
In Lightroom, backup your catalogue. Don't progress any further without doing this.
Now, in Aperture select a few images and use the command Metadata > Write IPTC Metadata to Original. So your keywords should be written to the files.
In Lightroom, select those images and use Cmd S - save metadata to file. So your adjustment work and other IPTC metadata is written to the files.
Then in Lightroom, Metadata > Read Metadata from File. This should bring in your keywords and not mess up your Lightroom adjustments. Review the results carefully. If it's worked repeat with the rest of your images.
Now abandon Aperture. Just like bigamy doesn't work for long, managing your photos with two similar apps is not good for you.
PS If you've not done much work in Lightroom, you may find it easier to start a new Lightroom catalogue and import the Aperture library into it.