Opening time extremely long

how can it be that image shows created with Fm5.x take so long to open. Partially up to about 10 times the time compared to FM5.x at about 300 images.
my hardware:
MacOs 12.0.1, iMac (Retina 5K, 27-inch, 2017), 3.4 GHz quad-core Intel Core i5, 16 GB 2400 MHz DDR4.