You can improve the performance of FusionAuth.
Like most applications, you should load test your particular situation. Will you have a lot of registrations? User searches? Logins? Will traffic be steady state or come in waves?
FusionAuth tends to be CPU bound when it comes to logins, because that's the goal of password hashing algorithms.
In general you can scale FusionAuth two ways:vertically. This means using a bigger server. horizontally. This means using more servers. FusionAuth is 100% stateless and can scale with as many nodes as you'd like.
Don't forget that FusionAuth isn't an island; make sure that the database FusionAuth is using is scaled appropriately, as well as Elasticsearch, if applicable.
Here are some specific numbers that may be helpful to you: https://fusionauth.io/community/forum/topic/8/what-level-of-performance-can-we-expect-with-using-fusionauth-as-an-idp
For detailed answers about your specific use cases, we recommend purchasing a support contract so that we can dig into particulars. You can do that here.