This is quite terrible in Safari (all latest) I just confirmed on not retina.
There is a browser bug with css animations, you can confirm it by disabling the animations and seeing the problem go away.
.discourse-no-touch .post-cloak .gutter .reply-new {
opacity: 1;
transition: none;
}
nav.post-controls button {
transition: none;
}
.discourse-no-touch .post-cloak div.actions, .discourse-no-touch .post-cloak .post-actions {
opacity: 1;
transition: none;
}