EA running terribly slow with same Symbol

As you know there is the "Symbol" you select at strategy tester and the "Symbol" you define in your EA. Normally they are both the same.
Today all of a sudden my EA started to work really slow. (I only use Open Prices).. I could not find the answer and I just checked if it would work fine at other instances. And it did.
For example the EA (which is originally written for USDJPY) runs really fast when used on other symbols but not with itself.. with itself runs terribly slow.
How is that possible?

MT4 backtester can't trade a pair different than the one selected.

If the EA is harcoded to run only on USDJPY, the backtester will do an "empty run" if you select any other pair. Hence it is fast ...

When you backtest on the pair it is coded for, it runs as it should.

I have performed hundreds of backtests with mixed pairs.. There is no problem with that.

I think the problem has sth to do with iCustom(xx). When I use a tech indicator embedded in mt4 the problem disappears..

Still what I cant explain is that this problem popped up only yesterday. I had been doing backtest with same settings for a long time.

I did a lot of optimization runs lately and been asking myself if it might sth to do with some cache issues or so..