New iMac very slow to render large TIFF files

My newer iMac (2020, 10 core i9, Radeon Pro 5700 XT, 64GB DDR4, Sequoia 15.0.1) is VERY SLOW in rendering large (250MB+) TIFF files in PREVIEW and all APPS, compared to my older iMac (2017, Quad Core i7,Radeon Pro 580, 40GB DDR4, Ventura 13.7). This issue is long standing, iMac in question has passed Apple tests at Apple authorized repair center, with a clean bit of health but the problem remains! Anybody out there with similar problem? I was expecting more from my newer iMac!