There is a trade off between retrieval speed and update speed when indexes are used, since each time a value in an indexed column is changed, the system must also update the index. Overall database performance can actually deteriorate if indexes are built on columns that are seldom searched but are frequently updated. You might want to build a number of indexes on a read-only table, but limit a frequently updated table to a minimal set of indexes.
When importing data into a table, it is significantly faster to create indexes after the data has been inserted. Prior to importing, you should drop all indexes that are not required to enforce uniqueness. Use a batch SQL file or procedure to re-create indexes after importing.