Dramatic Sessions Loss after new Theme Launch

Hi, I am hoping someone can help me understand what may be happening with my site. End of February we launched a new Shopify Theme for our store. Trying to make it more modern. Since then we have seen a huge decrease in sessions. The new site was showing as slow so I hired a Shopify expert to speed it up. They made the changes end of March. Is it normal for Google to take this long to resume traffic or is there something wrong with our site? Please help!