It appears that you have huge volumes of data coming in from different sources and please note tableau is a dashboard tool to show summarized data.I would suggest three ways initially to approach your problem until you figure out the actual problem.
1) to use extracts in tableau initially instead of fetching complete data from all sources.
2) please add few DB filters at tableau after you add the join condition between tables for each source and fetch limited records say 100 records.
3) pull in the needed fields but not all the fields into tableau
hope it helps.