flymake-mode is largely unusable in large Elisp buffers with checkdoc
enabled as it sometimes takes a second for the typed words to be
displayed, and there are too many micro freezes.  I have attached two
profiles one from emacs -Q, and one from my personal configuration saved
using the command profiler-report-write-profile.  Both point to
checkdoc-this-string-valid being the time consuming function.

To reproduce,

    1. src/emacs -Q
    2. C-x C-f lisp/org/org.el RET
    3. M-x flymake-mode RET
    4. M-g i org-drag-line-forward RET
    5. Move the point to be inside the docstring, and start typing away.
       You should notice the latency sooner or later.

I can reproduce this in my fairly long init.el file too (~10k lines) and
I was forced to turn off checkdoc for the typing to be bearable in my
init.el file.