HR staffing screened me twice. Then, the technical interview.

I was interviewed by about three or four engineers.

I was asked about compiler theories and concepts, such as software scheduling, register allocation, and loop unrolling.

In addition, some fundamental knowledge concerning the virtual machine runtime was discussed.

Then, we reviewed my resume and they asked a series of questions about the history of my past employments.

Finally, HR contacted me again to discuss the offer and salary.

The performance of a program is slower than the same program was before the compiler's backend changed. What direction would you look into to solve this issue?