I tested Workshop5 and Workshop6 and looks they already started running slow for all cores. I was able to get smaller script that have the problem. It do not only happens with multi core. On the script below with 2 cores the walk 1 and walk 2 runned fast. The walk 3 and walk 4 runned slow. The same happened after disabling multi core. I increased the number of cores and the first walk for all cores runned fast and it did make the walk train fast even for 2 cores and for no multi core. The speed up was preserved after a Zorro restart.
Commenting out "while(asset(loop("EUR/USD")))" while keeping "EUR/USD" selected did make this code runs slows. However with my script half cores started running fast and half slow.

Code:
function tradeTrend()
{
	TimeFrame = 1;	
	optimize(500, 300, 700);
}

function run()
{
	set(PARAMETERS+FACTORS);
	NumCores = 0;
	BarPeriod = 60;
	LookBack = 0;
	NumYears = 8;
	NumWFOCycles = 16;

	//while(asset(loop("EUR/USD")))
	while(algo(loop("TR")))
	{ 
		tradeTrend();
	}
}