Is Unraid the right choice?

Hi,

I'm breaking my head the last days to decide to deprecate my ds918+ with a custom built Unraid machine.

I will probably buy 4x18tb and would like to reuse (of course with full wipe) my existing 4 2tb drives.

I would use unraid mainly for running Seedsync (to transfer from seedbox to unraid) and to run Plex with my i7 8700k's hardware transcoding.
I've now read a lot that people report problems with Plex buffering on Unraid or Plex just not starting up anymore.

I also have the possibility to add NVME (100gb) as caching, or a 1TB SSD for the cache.

My main question:
I saw alot of people reporting, that they only get 40MB/s transfer speeds on Unraid because of the parity calc. I will probably never transfer files remotely to the machine, but seedsync will constantly download from a remote server. Will the speeds be way better than 40MB/s when I download locally on the unraid machine from a remote server (LFTP)?
Would you recommend Unraid for this Usecase? I really don't want to built a new server and then just recognize that Unraid was not the right choice :/