What is expected upload speed for this setup with large files?

CPU: Celeron g3930
OS: Debian 9 running under WSL on Windows Server 2019 on SSD
HDD: data directory on raid 1 HDD (150+ MB/s read and write)
Networking: Gigabit

As title says, I’m wondering what should I expect from this setup running newest NextCloud recommended optimizations while uploading large files (10+ GB).

For a benchmark on your system, you can transfer via SFTP. Nextcloud is using the database and the data goes through more layers which slows everything down. I haven’t done tests recently, but if you get less than 60% of the SFTP speed with large files, then there is probably some potential to optimize your system. For small files it is even more important, there the 4 GB RAM is not gigantic and on top of that Windows is eating a good part of it.