If you're referring to your Shopify speed score, it's likely the decrease happened by installing more apps - that tool used the Google lighthouse API, a general performance analysis tool that doesn't score well for engaging ecommerce sites or correlate to actual speed in some cases. More about that here if you want details: https://speedboostr.com/shopify-speed-tool.
I recommend using an analysis tool specifically for Shopify sites. Here's one that my team built free for the community: Shopify Analyzer.
That will tell you what can and should be optimized on a Shopify site (based on data from thousands of Shopify sites we've optimized).
It also gives recommendations and resources to optimize in-house. Of course if you prefer to hire it out you can just contact us through the site.