Slow screen redrawing in terminal emulator

Question

I use a modified PuTTY (PuTTY Tray, derived from PuTTYcyg) to connect to my Cygwin's login shell (zsh), on Windows 7 (64-bit). This works, but the screen refreshing is very slow. For example, when I open Vim, I can watch it scan down the screen. This becomes a particular problem when editing and navigating code.

My .zshrc is set to attach straight into a tmux session (or create one, if it doesn't exist). However, the problem exists outside of tmux when using Vim; when in the shell, it seems OK, but only anecdotally.

I've run this setup on Windows XP (32-bit) and it was never a problem.

My presumption is that the TERM value is possibly not set properly. However, I may be way off! Currently it is set as:

PuTTY Tray   Terminal-type string: screen-256color
             Terminal-speeds:      38400,38400

.zprofile    export TERM='screen-256color'

.tmux.conf   set -g default-terminal "screen-256color"

.vimrc       set term=screen-256color
             set t_Co=256

I've found that I need these options to get a working and consistent 256 colour mode. However, might this be the problem, or is there some other reason why the redraw rate is so slow?