Pausing the simulation allows you to return the speed to the minimum without additional button or function 
Otherwise,
add this function after the previous
function sc_simulate_speedup()
.....
end
Code:
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locate the appropriate navigatemap.ui containing the simulation buttons, find <layer ui_SimulationControls definition
copy the entire last button definition which contain sc_simulate_speedup() ad control speed of simulation ,
to add a 'decrease button' on left, put the previous copy just under <HBOX class="simulation_controls at first place in the uttons list and replace sc_simulate_speedup() in it by sc_simulate_speeddown()
save ui and lua, that's all
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