Goldfinches consuming sunflower hearts at alarming rate

Hi everyone, I was just wondering if anyone had any good tips to slow my usual group of goldfinches down? I'm currently using a robin feeder with a perspex cover that the blue tits visit when the goldfinches are sat at my main feeder, but would like an alternative to try and make the sunflower hearts last a bit longer. If I were to get a nyjer seed feeder and nyjer seed, would they go through that slower and prefer that? 

Thanks.