How to find source of these strange SQL queries?

They’re from WordPress. They come from the wp_old_slug_redirect() function which is run whenever there is a 404. The purpose is to check if the requested URL was the old URL for a post so that it can redirect to the new URL.

If you’re seeing a lot of these then it means you’re getting a lot of 404 hits. The slugs in your example suggest that it’s probably bots, but it’s hard to say. I highly doubt this query alone is causing significant server load. It’s probably the volume of the requests themselves that is the problem.