I\'m running into a problem in a Magento system where saving a large number of attributes either doesn\'t work at all, or only partially works. It appears to be a javascrip
I had exactly this problem (POST truncated) and it comes from a suhosin patch that has a too little POST limit. (or the standard PHP post_max_size)
In your php.ini check these values and increase their values if needed (and restart apache) :
post_max_size
suhosin.post.max_vars
suhosin.request.max_vars
For your second probleme (JS performance issue), I can't help you