You're not going to get an apples-to-apples comparison between Synthetic and LUX, because they work differently. It's quite typical for RUM metrics to be much faster than Synthetic. There are a number of reasons for this, the two most important being:
Browser caching – Synthetic tests your pages as first-time views, with cold browser caches. For actual users, some amount of caching will occur, which can make pages considerably faster
Synthetic snapshot vs real world experiences – Synthetic tests are a snapshot of performance based on a very particular set of variables. RUM shows you the full breadth of actual user experiences, which involve users who have different contexts (e.g. browsers, devices, connection types, geolocations) than what you've set up in your Synthetic tests.
Having said all this, one of the benefits of having access to LUX and Synthetic data in the same set of dashboards is that you can use your LUX data – such as the data in your LUX Users dashboard – to create better Synthetic tests.
The most important thing to track is consistency and changes within a single tool and settings.
If you see changes in your results over time within Synthetic or within LUX, that's noteworthy. If you see differences between the results you get using different tools, that's not especially meaningful.