The big issue with native lazy load currently in Chromium is the thresholds values used: images start to be lazy loaded only if they are 3000 to 8000 pixels below the fold, depending on your connection type: https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/settings.json5?l=971-1003&rcl=e8f3cf0bbe085fee0d1b468e84395aad3ebb2cad

These values are huge, especially compared to what is currently used by most lazy load plugins out there.

If someone was not using a plugin for that before, it will go from no changes to a possible improvement, but If they switches from a plugin solution to the native solution after 5.4, they might in fact face a decrease of performance and loading time on their website, instead of an expected increase.