Although this thread is 3 years old, I'll answer anyway. I use IB and trade ES. It's not 0.25 slippage. If you look at your trade report the execution price is the price traded. Almost always this is my limit price. However, the price in my account gets adjusted with the commission I paid. This is mostly 0.25 lower when shorting and 0.25 higher when long. They do this, so you know your profit/loss after costs. Also, when I used to trade stocks the adjusted price was a lot more than 0.25.
If he was trading lunch time which I know few traders that only trade at lunch, and if you using stops to enter ES, you can get slippage on half your trades, but best to use stop limits, might miss the trade but if one is scalping and targets too low, it is better to miss trade than give up a tick. I rather give up a tick on some signals as they go after more profit than others. But most other scalpers I know go much less than I do of 2-3 ticks.
Try looking at software like R Trader by Rithmic for ES, because it actually measures the time of execution from your machine to the exchange. Maybe a change of hardware and speed of net could potentially improve your execution.