Not that bad. I have 32ms directly to Wafflepool. But the proxy itself will also add some latency. And if you are using getwork then the notifications about expired/new blocks won't reach your miners. Even if longpoll is supported and used, compared to stratum it has a higher chance to "deliver" the block expiration notification too late. And this can become significant in a pool with a very big hashrate. In the last 10 minutes there was minimum a dozen new block with just 1 second difference to the previous one.