Picture (which is big in aspect ratio) is really bad quality when opened in photoshop(smaller aspect ratio)

So I have this image which is 5312 pixels in width and 2988 pixels in height. I need to create a banner ad in this ratio: 300x250.

So this is what I do:

1) Create a new Photoshop document in this ratio 300x250.

2) Drag the 5312x2988 picture into the Photoshop document. The picture when comes perfectly fitted into the white box, but I can see the quality is wayyyyy lower. I still need it a little bit smaller though.

3) Hold shift and then make it a little bit smaller and then the quality gets even worse.

Can anyone help me out how I should open this picture in this banned ad ratio in perfect quality Should be possible in 2018 right?