Assets at sketchfab or turbosquid are often not “game-ready” since the poly count and/or texture resolution is way too high.
It’s really hard to quantify proper poly counts and texture resolutions since it massively depends on the use case. But for reference, even modern PS4 games often have just about 100.000 polygons per character model. In context of mobile games, it’s of course much less than that. Same for textures. You normally don’t want to use 2048x2048 textures for mobile games since the memory allocation for a single map and the respective mips is way too high.
Besides, you should always use
glTF as your primary 3D format instead of e.g. OBJ or FBX. Using
glTF will produce smaller files, especially if you DRACO-compress mesh data and/or use texture compression. In this way, you can massively shrink the files sizes to something manageable for the web.