Zoom Effect Is Jerky

Hi, I work with FCPX 10.4.8 on iMac, macOS Catalina 10.15..4, processor 3,6 GHz 8-Core Intel Core i9, memory 64 GB 2667 MHz DDR4, graphics Radeon Pro Vega 48 8 GB. I'm trying to do a zoom effect on pictures using keyframes and cropping (under shift/C). The pic is at first smaller against a background. I make a compound clip using option/G and zoom it in so the pic finally fills the entire screen. Works just fine except the zooming is jerky: zoom-slow-zoom-slow... nothing like a camera zoom. Is it my Mac (has been a little weird since Catalina) or FCPX or something totally different? Thankful for any tips.