Are the insertions happening in a transaction? If you perform single INSERT INTO statements in auto commit mode it will be very slow, as each of the individual insertions will be written to disk. Bulk insert statements are much faster. If you have to load a very large dataset, you might want to use COPY INTO rather than individual INSERT INTO statements as well. Check out this page for more information and performance tips:
https://www.monetdb.org/Documentation/Cookbooks/SQLrecipes/LoadingBulkData