It's probably building Android and Gradle models, that can take some time. There are two parts of sync:
1. Gradle part: project configuration
2. IDE part: building "Gradle models" and "Android models": basically collecting all the data that IDE requires as far as I understand
There were some improvements to do 2. in parallel in the Android Studio for Android models if I remember correctly
https://developer.android.com/build/releases/past-releases/agp-7-4-0-release-notes. Not sure which version of AS supports that, but the latest the better.
There is also a flag in IntelliJ 2023.3 I think to also build some Gradle models in parallel, see
Enable-Parallel-model-fetch-by-default (it also requires
org.gradle.parallel=true
I believe).
There will be also improvements for 1. with
isolated project, which will basically allow parallel configuration, first for syncs and later also for task execution.