There are many reasons why build duration can be higher when using a build service:
When running your build locally, build tools may cache source components (e.g. NuGet packages, pods, dependencies) to make subsequent builds faster. App Center configures a clean VM for each build, and therefore must download all dependencies before starting a build.
For Xamarin builds, when running your build locally, you are most likely running a simulator build; In App Center you can run a device build (signed), which takes much longer to run.
The CPU power of your local build system is higher than the CPU of the cloud build VM.
We are always working to improve build times. If you consider the build duration for your app is too long compared to your expectations, please reach out to us via the in-app chat (Intercom) or with a comment here.