Slow scanning on Android is a known issue on various modern Android versions (android 12 extremely slow, How to deal with abysmal I/O performance on Android 10+?) and has nothing to do with hashing. Rather, with Android being Android, you get abysmal performance when not going through Android’s preferred I/O path.
In general, raw SHA256 shouldn’t be the bottleneck on a modern smartphone processor. My Galaxy S10e Exynos processor (from 2019) does ~800 MiB/s SHA-256 according to syncthing’s internal benchmark - far faster than the internal flash can deliver.
Syncthing already has a weak/rolling hash (to detect shifted blocks), but it won’t help you for the initial scan - the initial scan needs to read the entire file and fully run cryptographic hashes over it for architectural reasons - syncthing needs this for data integrity across devices.