I´m editing a video with photos, suddenly FCP X displays the spinning ball a few seconds every time I play the Timeline

I´m in a project that is primarily composed of photos, lots of them. When I reached 10 minutes of work the spinning ball started to appear after an edit and when I try to play the timeline to see the results or preview my edit. I checked the source material and I realized that I made a begginner mistake: I had a lot of photos with too high resolution (4000x3000 for example), so I resized all photos to a more manageable size for FCP, 1024x768, and then I saw the light! FCP began to work normally again. Later I added more photos to the events folders, and when I finished the import, FCP returned to the same thing I described before. Someone has a Tip? I am working with too many photos? the video is planned to last 20 minutes, maybe I will assemble it with 400 photos, but in the events I have lots of photos, 2600 aprox, some of it referenced and others copied in the events folders. Maybe I have a lot of photos in the events folders?