you can mitigate some of this by prioritising ACKs on the up link.
There has been quite a bit of discussion about doing this with OBSD's pf
firewall on the pf mailing list.

Russell

Philip D'Ath wrote:
> Ran into an interesting problem the other day.
>
> Had a business customer that was on full speed JetStream. They were
> forced to move to UBS like everyone else, but liked the speed, so
> decided to go for a 2Mb/s plan.
>
> They gave me a call the other day complaining their WWW browsing was
> very poor. Assumed it was related to the ISP, but decided to
> investigate further.
>
>
> Turns out the problem occurs when a small number of users all decide to
> send an email at the same time (they don't have an internal mail server,
> so use their ISP's SMTP server). This chokes up the 128Kb/s upstream,
> and was causing the TCP ACKs to be substantially delayed, causing the
> observed slow down in WWW browsing.
>
> Did a further test. I've I get several of their users to all send an
> email while doing a download I found the download throughput dropped to
> a poultry 64Kb/s. As soon as the emails finish speed takes off again.
>
>
> There's been some talk about how the 128Kb/s upstream limits the maximum
> performance. However, the problem is actually much more severe,
> especially when many users sit behind such a link. Very quickly you
> reach the point where even a small business can't possibly use the 2Mb/s
> which they are paying for.
>

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