I've always struggles with iambic pentameter. I know what it is but the de-dum de-dum explanation doesn't sound anything like what I hear in performances, origi

I've always struggles with iambic pentameter. I know what it is but the de-dum de-dum explanation doesn't sound anything like what I hear in performances, original pronunciation or not. Rather than a galloping rhythm, actors seem to leave cavernous pauses in the middle of lines wherever they like. There must be a reason for it, or it wouldn't be included, but what is the effect it's supposed to give?