The sys schema helps you interpret the data collected from the performance_schema in an easy and more understandable form. performance_schema should be enabled for sys schema to work. To use the sys schema to its fullest extent, you need to enable all the consumers and timers on performance_schema, but this impacts the performance of the server. So, enable consumers for only those that you are looking for.
A view with the x$ prefix displays data in picoseconds, which is used by other tools for further processing; other tables are human readable.