The file has a size of 6280x1960px. I tried smaller sizes, I tried larger sizes, nothing works. The quality always looks bad. Is there an ideal size I should use for this that might help?
Just wanted to let you know that I was able to fix the issue. It was caused by the image-rendering property on the logo (image-rendering: -webkit-optimize-contrast;).
This additional CSS fixed it:
.site-title img {
image-rendering: auto !important;
}