Merge as new vs inline merge PQ

As I am getting the hang of PQ and PP, I'm starting to look at optimizing queries since they're now all over the place and seem to have quite some loading times.

tl;dr is basically, are there pro's and cons in using inline merges vs 'new merges'?

Small example;
External Table 1 is inline merged on External Table 2;
External Table 1 is inline merged on Local Table 3;
Local Table 3 is joined on External Table 2;

How often is External Table 1 retrieved? One time, after which its data is merged 2 times? Or it is retrieved at the initial connection, and each inline merge at the 2 other queries?

Would it be better to load External Table 1, External Table 2, and Local Table 3, and merge as new, which then is loaded into data model? Or, is there really no difference, and is data loaded only one time anyway?

Thanks in advance for your time.