Applymap is definitely faster and is recommended by qlik when only 1 column is involved.
check below link
https://community.qlik.com/t5/Qlik-Design-Blog/Don-t-join-use-Applymap-instead/ba-p/1467592
technical reason i dont know if its documented anywhere. but my understanding is that because its a lookup during your load of your initial table. e.g.
in case of join you have to load table1 into memory, load table 2 into memory and then the join is performed
for applymap load table2 into a map, table 1 into memory (with applymap performed during that load).
Small datasets you may not see much difference but for bigger datasets you will definitely see the difference
