Unless the hard disk is nearly full, the quantity, nature or size of files (of any type) stored on it should have no impact on the PC's normal responsiveness.

If the disk is nearing capacity, be sure to clear temp files, cache folders for browsers, etc.

in all likely hood, it will have no effect at all.

Stored files (photos, document, music, videos, etc.) would not affect computer performance because they are not interacting actively with the hard drive unless they are in use. If you had 20,000 pictures with an average size of 2MB, that would only be about 20 GB total. These days, an average computer has a hard drive that is 500-1000GB. That means that those photos only take up 2-4% of the hard drive capacity. The most important thing is that you are backing up those photos elsewhere for when the hard drive inevitably crashes.

Thanks for the helpful replies.

I store my photo's, slideshows etc. on flash drives and also at online storage sites one of which gives me 50GB free, that's a lot.