Poojitha Puligundla Thanks for posting your question in Microsoft Q&A. I assume you are already aware about Optimization guide for Power BI doc and its recommendations in general. From the description above, it looks like you are focusing on the call from Azure vs directly calling Rest API since it is taking twice the loading time.
There are several factors that might affect performance such as network latency, capacity of the azure function (CPU/Memory) depending on the plan, react application code etc. I suggest start with Application Insights Integration as described in Monitor executions in Azure Functions doc which will help with performance counters, dependencies, network latency etc. If you observe slowness in the initial run, this could be related to cold start especially on the consumption plan (https://azure.microsoft.com/en-us/blog/understanding-serverless-cold-start/).
Based on analyzing and monitoring logs, you can review scaling or optimize code for handling requests. It is also good to verify the react application performance in a different environment (or local/dev) and compare it with directly calling Rest API.
I am not expert in PowerBI optimization and PowerBI is currently not supported in Microsoft Q&A forum. If you have any specific questions related to PowerBI, please post your question in https://powerusers.microsoft.com/t5/Power-Apps-Community/ct-p/PowerApps1 and community experts can assist you further. I hope this helps and let me know if any questions.
If you found the answer to your question helpful, please take a moment to mark it as Yes
for others to benefit from your experience. Or simply add a comment tagging me and would be happy to answer your questions.