While there are not technically maximum limits on the number of images you can store in the Image Library or display in Galleries, performance will begin to decrease after certain numbers are reached. This has to do largely with the file sizes of images that have been uploaded. Internet speeds and computer processor power are also factors in performance.
While no actual limits are in place, if your images are 1 MB in file size (for example), having 500 images (or less) within the Image Library should not result in any issues. If your file sizes are smaller, you may be able to store more images within your Image Library without performance problems.
Image Gallery blocks that use large image display layouts (Slideshow, Carousel, etc.), we recommend a maximum of 20 images. For thumbnail type galleries (Grid, Vertical, Mosaic, etc.) 50 or fewer images are suggested. If the viewer’s computer or mobile device is newer (i.e., has a more powerful processor), some more images should be fine.
Overlay effects (Color Mask, etc.) should have little to no performance impact on any device. Transformations like Zoom, Focus, Shake, and Rotate should also be unlikely to cause any loading or performance issues. The effects that use CSS filters (Grayscale and Blur) have some potential impacts because those are still somewhat new, even experimental CSS features and they’re fairly processor and/or GPU intensive. If you are experiencing loading problems or slowness and you have either Grayscale or Blur enabled, try adjusting those settings to see if this is causing the issue.