A quad core processor is a unit that contains four cores meaning that the performance will be better than a processor that only has 2 cores inside. Also, the performance on processors depends on the platform technology, features and processor model.
Based on your question, you may find some of the most important differences between an i3 and i5 processors:
1. Intel Turbo Boost is a technology that can be found on Intel® Core i5 & i7 processors only. It is important to mention that this feature helps the processor to increase its clock speed (provides a boost of power) when the system needs it in order to complete a specific task.
2. You can find the i3 processors with 2 cores and 4 threads. In the other hand, the i5 processors can be found with 4 cores and 4 threads.
3. Intel® Hyper Threading Technology is available on i3 & i7 processors only. This feature can be available on i5 processor for portable versions only (processor laptops). This feature uses processor resources more efficiently, enabling multiple threads to run on each core. As a performance feature, Intel HT Technology also increases processor throughput, improving overall performance on threaded software.
4. The higher the number, the better performance on processors. I mean that i5 will have better performance than i3 and i7 processors will also have better performance than i5 & i3 processors.
Hope this is helpful!