Multiple lag/glitch issues specific to 15w39c and later

So whatever changes were summarised in the changelog for 15w39c as "improved performance on older computers" have significantly broken MC on my system (tested under both Linux AND Windows with identical issues; I'm guessing these changes are the culprit.) These issues did not exist under 15w38 and before; unfortunately I didn't get to extensively test 39a/b but I did not experience these issues under 39b for the short time I tested it.

-Significant block lag for up to 4-5 minutes after spawn (always been an issue, but is much worse now)
-Mobs invisible for some time after spawn (but will still attack and kill me, making surviving outdoor respawn at night almost impossible)
-Visual glitching (invisible ground/near chunks invisible) at spawn, probably exacerbating the lag
-Overall, massive lag after spawn/respawn; much, much more than is usual, to the point where the game is unplayable.
-**Worth noting: this does NOT seem to be directly related to chunk updates, unlike normal spawn block lag present in all other versions. Lag, visual glitches, invisible mobs continue to occur after all local chunks have completed loading.**
-Lighting lag when placing torches - lighting takes a significant fraction of a second to update when a light source is placed; in all previous versions lighting updated instantaneously, even when lag was otherwise present. Has lighting been moved to server-side?

These changes have more or less broken my favourite game! I'm far too spoiled by new features to revert to 1.8 now! PLZ UNBREAK!