I have been with lightspeed since they merged with schonfeld. I prefer schonfelds old platform better than lightspeed with my main gripe being execution. Alot of times on lightspeed I am paying through the market. This did not happen on schonex (the best platform IMHO), lightspeed has a smart route key that gets around this but my executions are still not as good as schonys old patform. I trade over 10,000,000 shares a month so these bad executions add up.
As far as speed lightspeed is pretty fast most of the time but again not as fast as schonfelds old platform. For comparison I ran both of them next to each other for a week when I had access to both platforms. The funny thing was after talking to tech support they told me there shouldn't be any speed difference because all the data was coming from their same servers. Anyway smart order is best most of the time but when you are trying to get out of 5000 shares of skf I recommend using rash through the market.