Global variables - slippage with playback

I try to backtest a spread strategy with global variables with very typical setup - chart1 is master with algo in it and chart2 is slave which simply takes opposite position vs chart1. When using playback i turn on global mode. All data is 1 minute and when i start to speed up playback I notice slippage in trades. Basically i get the following results:

up to 100 updates every second - no slippage
above 1000 updates every second - massive slippage and no sync with two charts sharing global variables,
at max playback speed (10000) - slave is not putting any trades at all

What is a solution here? Is playback suitable for backtesting global variables?

I::

Is playback suitable for backtesting global variables?

No playback is NOT suitable for all types of backtesting.

One reason: playback is not a tick-by-tick replay of the data.