When I join to a server or i play in my world, the World Generator Lags (Sometimes)

When i Join to a server, I saw the map of the host with many mountains around them, so.. like 2 minutes after being in the map, many mountains start to dissappear, and it was replaced by trees, or smaller mountains.In fact the world generator lags, also in 0.7.4 now its affecting when you play on your world, it takes more time to load the map.

Here is a video proof showing that Bug: http://youtu.be/Nuwr7Y4ejtI?t=5m21s