I already said, lookup_ext() as a new-function-call is preferred as it can bring up the lookup wizard again whereas as a mapping all is textual. But there is another impact, a performance impact.
All mapping functions of a query transform are executed in one thread, one after the other. As a result, if you have 10 lookups as mappings in one query and your server has 10 CPUs, all processors will be idle except one. (Assuming the DOP is not used see Degree of Parallelism ) So you would need to have ten queries, one per lookup to get 10 parallel threads, each using a CPU on its own.
If the query has the 10 lookups as new-function-calls, the DI optimizer assumes these to be rather heavy computing processes and will automatically have one query-thread per new-function-call.