I think it's been a bit laggier in general, but that could just be me. With respect to your unopenable chest, did you have any mods installed before the update? Pretty much every mod gets broken every time there's a game update,and if you have any mod-related items in chests the game crashes when you try to open them. If this is the case, you'll just have to wait until whichever mod it was gets updated. If that's not the problem and I'm just being patronizing, I really don't know what you can do other than, again, wait for a bug fix.
So, yeah, every update's a bit of a pain anyway, to be honest, having to update all your texture packs and any mods you use. For example, I was using a pretty extensive mod which adds villages to the game, and when I updated to 1.6 all of the npcs, like, evaporated. I reckon it's worth it, though. Probably.
There's actually a quick fix you can try. Find a copy of the 1.5 .bin folder and replace your new 1.6 .bin with it. That'll revert you to beta 1.5, meaning none of the new stuff for 1.6 will be in your game, but you should be able to use stuff as you did before the update.