[Daynarr]

Ok, try 2 things in this order and not necessarily at the same time.
First try going to full screen to see if it helps (alt+enter is fastest way). Means, go full screen and then start battle replay, not sure if it helps if you do it during battle replay.
Second thing to try is to disable light effects in video options. It has huge performance issues on some older video cards and it may give you much better performance if you disable it.