Step one is to understand what is going on. Use WebPageTest, or Lighthouse (or any other tool) to figure out where the pain points might be. Some sites might suffer from JS bloat, others on images.
Use monitoring tools like SpeedCurve or RUM tools to understand where your customer's pain points might be.
Then, once you understand the problems...you can roll up your sleeves and start fixing the code.
Also, test on low powered devices, and slow connections. Get an Android Go device. Slow down the network speed to 1.6 MBPS. Then test. As devs, we tend to have fast devices and top notch internet connections, but not everyone does.
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