"It won't do to say that the reader is too dumb or too lazy to keep pace with the train of thought."

It won't do : a particular person's behavior is unsatisfactory and cannot be allowed to continue.

1. = bewilder / confuse completely 使困惑

you are  mystified by something = you find it impossible to explain or understand

2. =derogatory / obfuscate 将……神秘化

With each rewrite I try to make what I have written tighter, stronger and more precise, eliminating every element that's not doing useful work.

Clutter is the disease of American writing. We are a society strangling in unnecessary words, circular constructions, pompous frills and meaningless jargon.

上面两个句子说的是简洁精炼语言和冗长的语言。学习其表达。