It is normal to have low frame rates when you run 3DMark. It does not mean there is something wrong with your PC.
Benchmarks are designed to place a heavy load on your computer. A heavy load makes it easier to find the bottlenecks in your system. It also extends the usefulness of the benchmark by ensuring that it will still be relevant for future hardware generations.
Particularly in 3DMark, it is sometimes the case that only small differences in visual quality can be seen from very large increases in the amount of processing required when using the highest settings.
Rather than worry about low frame rates, it is better to focus on how your 3DMark benchmark score compares with results from similar systems. The 3DMark result screen helps you answer the question, "Is this a good 3DMark score for my PC?"