It would be beneficial in terms of usability, if multiple joins could be done in one transform.
Often there are multiple sources where values have to be retrieved from. Currently you either have to do multiple joins one after another to gather all the values or you have to define several lookup functions in a field transform. The latter seems to be less performant than table joins (when multiple values/columns of each source are needed).
A new type of transform should be introduced where multiple joins can be done. One source could be joined with multiple other sources. For each of the other sources the join type and columns should be defined.