Try to play with compressor.
In oc-includes/osclass/frm/item.frm...
find this line:
}).use(Uppy.Compressor, {
I found it supports maxWidth and maxHeight, so if image is larger, it's going to convert to smaller dimension.
This can be handy if you upload image 8000x8000px, but on site you generate just 1024x768... you don't need that big one.
Change it to:
}).use(Uppy.Compressor, {
quality: 0.85,
maxWidth: 1600,
maxHeight: 1600
Carefully if you use "keep aspect ratio" as you might get portrait image... but if you use for maxWidth and height same value, you should be OK.
I tested it and it do compression on upload, so PHP already receive smaller image and might be faster.
This will not work for Importer as this one provides images to osclass diferently