First, re the idea of images and FTM performance. FTM does not include media in its database for the reasons you are concerned about. FTM keeps its images in a separate folder which is only accessed if you wish to take a look at the item. Otherwise all that is in the database is a link to the image.
Secondly, if you decide to link images from FTM to wherever on your hard disk they are located, you risk future problems. Five or ten years from now, you may have forgotten about some of these images being linked and moved them, or worse, deleted them, or had a hard disk crash and recovered the FTM file and not the images. I strongly recommend you ATTACH your images (including .txt and doc and pdf file) into FTM so that a copy is placed in the FTM media folder.
Thirdly, your file is small enough that any performance reduction that exists shouldn't be noticed at all.
You access the media tab in the Source Workplace by double clicking on a citation. Once you have the media item in the Media Folder for one citation - you would link to it after that point with other citations.
Secondly, re repositories. The repository for items you might have in your possession, is where you, or another researcher, can get the item. Since most items can be, and are, accessible via going to the source - there is no need to show yourself as the repository. That would be intended for such things as you have the only copy (or original) of a letter sent by your 3xg great granddad to 3xg grandma.
For example, one place where a birth certificate might be obtained is the Board of Health of whatever state. Using that board of health as the source already divulges the repository. There is no need to repeat it in the "repository" box. Also, common, or standard, books don't need a repository (eg Savage's New England Dictionary or The Great Migration or etc.)
Remember that info in the repository box is printed in the reference note for your citation - so you want it to be as brief as possible - or omitted if it is self-evident by the name of the source (ie OhioOn-LineVitalStatistics.com or whatever).