I am using high resolution images
What size are the images, in pixels? If you have say, a 24 megapixel camera then the shots from that will be 6,000 pixels x 4,000 pixels. When you export them in a slide show the images are compressed severely: down to 1080 x 1920 - which is a little over 2 megapixels. So there is always the risk of significant quality loss.
That said I've not found the loss to be terribly noticeable so perhaps you might try and compare the results with some other apps - even on free trials - and see if the result more suits your taste.
Alternatives to Photos slideshow include:
iMovie is on every Mac sold.
Others, in order of price:
PhotoToMovie $49.95
PulpMotion $129
FotoMagico $99
Final Cut Pro X $299
It's difficult to compare these apps. They have differences in capability - some are driven off templates. some aren't. Some have a wider variety of transitions. Others will have excellent audio controls. It's worth checking them out to see what meets your needs. However, there is no doubt that Final Cut Pro X is the most capable app of them all. You get what you pay for.