Emacs (Windows version) slow when transferring large amount of text over pty or pipe

[speech . free]

I am debugging a clang based completer. The completer running as a sub-process 
sends a large amount of text (about 1.8M) to emacs over a pty or pipe 
connection. The emacs connection buffer is about 4k, as seen from logging, that 
is the process filter is called for every 4k of text. The text is inserted into 
the process buffer and accumulated there and an EOT string is repeatedly 
searched for. Receiving the whole transfer takes close to 30 seconds, rendering 
the completer useless. The same problem is not seen on Linux. Any suggestion on 
why this is and how to deal with it? I have GNU Emacs 24.3.1 
(i386-mingw-nt6.2.9200)
 of 2013-03-17 on MARVIN.