I dont know what is going on but I am trying to make a path up to around Y 190+ and as soon as I get that high my client hits 99% CPU usage. Normally in my base I am around 6% there are no crash logs because the client never crashes just seizes up so I am unable to move. I am forced to shut the client down and reload as soon as it loads up I go back down and CPU drops to normal. I wanted to build my base in the sky so I can make better mob farms and more room to build in without being constantly attacked when my back is turned.
You might still be able to get a console log after a forced quit. Having that to examine might help determine what's eating up the CPU.