I can recommend writing in finance and risk management. You may relate performance to effective risk management in banking or in insurance or in any other financial institutions.
To check for a good topic for your thesis, I suggest you do the following:
1. Choose a number of research papers related to, say, Finance or Management (or may be even specific in this regard) from Google Scholar, not from any other source such as Google, etc.
2. Read these research papers and check for the research gaps from within such research. You may find these gaps under the subtitle, Future Research or similar subtitles.
3. Make sure that such gaps are not covered by recent research. That is, for the gap that you want to cover, read to the last current research in this regard to make sure that no other author(s) already conducted such research.
4. Then, go ahead and start preparing for your research, preferably using the Literature Review Matrix where you put a table showing authors, title, objectives, problem statement, variables, methodology used, findings, etc. for each research paper selected. By the way, you may want to keep such relevant research papers to use them as references in your research.
A good PhD dissertation topic is to investigate the sustainability of FinTech in the long run given the ongoing COVID-19 and climate change challenges. Can FinTech survive COVID-19 and climate change challenges in the long run?