Generally I have found that if the data has to be manipulated in some way, it's better to suck it into a temporary table, then manipulate it as desired then import from there to the destination table. If the data is going to be used "as is" and doesn't need to be manipulated in some way, I generally just import it into the destination table. A couple of factors affect the performance of imports :
1) Stored calculation fields. The more complex the calculation the slower the import.
2) Indexing of fields. Any field that is indexed has to update that index as the data goes in. I sometimes use "multiple key values" for things like search portals, and these really slow things down.
3) If running standalone (i.e. not from server) then raising the cache settings in FileMaker Preferences may help performance. This is a case of "give it a try and see".
Hope that sparks some ideas.