Perhaps you can start with comparing 1,000 individual INSERTs (each with its own COMMIT) vs. 1,000 INSERTs wrapped in a single BEGIN/COMMIT block. This test is particularly interesting for comparing different RDBMS because it reveals how each engine balances data durability (ACID) against performance. You will likely find that some DB have a much higher tax on commits than others, which is a key factor in choosing a DB for a specific use case.

Since you are in C, please make sure to use prepared statements. If you send raw SQL strings, you are partially benchmarking the string parser of the DB rather than the actual data manipulation. Also, remember to "warm up" the DB with a few hundred runs before you start the timer, otherwise, the results will be skewed by the initial disk-to-memory cache loading.