When you open finder on the left side of page under devices at the bottom there is a icon (Images).
I clicked on it and it looks like every image that has been on my computer has been saved there.
The answer to your question is "no, they don't slow down your computer ... until the hard drive is full."
If you're like most people, you can store many tens of thousands of images on your computer before it gets full, so it's likely not going to be something you have to worry about unless you are a crazy picture-taker or movie-maker!
You can always check how full your hard drive is by opening any Finder window and looking at the bottom center:
If your hard drive's free space ever gets below that number, it's time to look into moving your media collection to another drive or consider buying a bigger hard drive.
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