Rankings keep fluctuating #3 to #7

Hello, I’m writing to ask if anyone has any insight about why my ranking for a major keyword flip flops between 3 & 7, used to be constantly at 3 but since I updated my shopify theme few months back, it seems like Google can’t make up its mind about what position I rank for. I’m currently working on optimizing load speed and checking other core metrics, I believe my biggest hurdle is the heavy JS loading I have on this new theme that’s slowing it down.