There are a lot of metrics to describe speed. It can be tough to remember what all of the metrics are and how they relate to each other.
My suggestion is to quit thinking about the metrics independently. Instead, think of the metrics as telling a story about how the website loads. The story starts with the initial website load, moves into the middle stages as content is loaded to the page, and then moves toward the end as the website finishes loading.
Here is a rough timeline mapping out how the metrics fit together:
The website loading timeline begins with Time to First Byte (TTFB) which describes the very early stages; it is the start of the story. We move into the middle of the website load with First Contentful Paint and Largest Contentful Paint. The website finishes loading with metrics like Time to Interactive or Total Load Time.
Seen this way, you don’t need to know all the details about each of the metrics. Instead, just by understanding the general timeline, you can understand how the metrics that describe the later stages depend on the metrics that describe the earlier stages.
Once you understand the timeline, it becomes easier to know where to focus. If the first part of a timeline has problems, start by fixing those problems before even looking toward the later part of the timeline.
For more help understanding this timeline, and knowing what the metrics are telling you, check my book, Speed Metrics Guide. Published by Apress, it walks through the timeline and helps you understand what the different metrics measure and how those metrics fit together.