This is the back 4( + 1 sub teen player) i used for like 2 seasons and they are offside machines ( i can do offside like 3 or 4 times every match ), from my experience i can say 100 % Positioning matters for offside( and great GK Communication ) but speed is also useful if the oppo break the offside ( you will note this against defensive formations ), even if they break your trap your defenders will chase his players and recover the ball ( i think you already see that behaviour during simulations )
The first one is a sweeper , the man that guide the defence