Strange render time

I have a project I’ve been working on for a long time. It’s a static shot of a person in the frame. I don’t change the project template (I leave the music, transitions, color, and sound effects as they are).  I only change the still footage files. I used to render the project in 5 minutes; now it takes 25. I started investigating and realized that the transition file was slowing down the render by 300%. The file itself is short—a standard 1920x1080 MOV file lasting 3 seconds. I cleared the cache, updated to the latest version of Premiere, tried creating a new project, and experimented with different render settings—nothing helped. A file that had been working stably for many months has now started to slow down the render time. Why is this happening? What did Premiere do to this file? How can such a short file slow down the render time so much? This must be a bug on the developers' part.