Some areas are creating huge amounts of light calculation lag

(Possibly related to MC-12596)

In a particular area of my minecart railroad, I keep getting a large amount of lag. 'root.tick.connection.checkLight' keeps eating up all the CPU time, resulting in video for half a second, followed by two seconds of video being frozen.

The area has not always behaved like this.

The associated save has you travelling south by minecart, nearing a right turn to the west (which heads to my base). The lag seems to occur mainly on the north-south segment, though orientation probably has nothing to do with the issue.

A good amount of the rail is underwater, though the lag also occurs above sea level.

I believe the minecart is unrelated to the issue - IIRC I once got out and still had the lag.

The savegame, too large to upload, is located at

http://cdusto.selfip.com/MC-13505.7z