How to handle large amout of images for animation? (it loads and build very slow)

Hello, I've run into somewhat of an inconvenience that i'm positive there must be a better way than what i'm doing..

I've got a game, that i got assets for, in the form of like 300+ images for animation, that are around 550x550 dimensions.

After i've add them to the unity project, it takes a long time to load on startup, and a LONG time to build. I've tried making a sprite atlas as well by giving tag names to batches of sprites.. not sure how much that helped if it all.

Is there anything I can/should do to improve the situation?