How Does Ctrader Handle Closing Large Volume Buy and Sell Orders Simultaneously?

I am just testing a Cbot that at times will simultaneously close  a large volume of buy and sell orders.

Would I see a lot of slippage? or will ctrader  or my broker offset my orders so i do not see much slippage?

Example:

close  12 Million sell order

close 16 Million buy order

In a relatively slow volume period.

would these 2 orders offset right at my broker therefore encountering no slippage except for the final 4 Million?

Does anyone have any realtime live experience that could tell me how a transaction like this would be handled?

Thanks