I would recommend Python as the programming language and as you are a new developer, Flask to start with. It gives you a solid understanding on the web patterns such as REST and will get you up and running in no time. However, I suggest you to read and study on front-end technologies like (React or Vue) and databases (SQL and NoSQL) and probably some NodeJS as well. First grasp the concepts (which Python is ideal for) then it does not really matter the language as such.
I think it is not exactly important which language or framework for a high-performance application, but rather the architecture. Check where you want to deploy your system and how are those environments support your tech selection. For the backend, if you go with microservices you can use an array of languages for different MSs which you can choose based on the use case. Such as, if it's ML-based then Python, if it is super asynchronous you can use Go or NodeJS so forth.
For the front-end think of the community support as Angular is a bit lack in that sense, think of the team you have and their experience, I think you can have a look at VueJS and React as well, given the community.
