How to debug when vim becomes quite slow?

When i open a file with 4000line code, the response of vim becomes quit slow. At the mean time i input a word, vim costs 40% cpu. After switching to another vimrc, it works fine again. 

Can anyone tell me how to debug in vim? Because the vimrc script is written by me and i can't locate the error after checking for a long time.

Thanks in advance.

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