Re-posting the information that I posted on technet:
Parity Storage Space so slow that it's unusable
Folks experimenting with RS5 parity write performance on Server 2019/ Windows 10 RS5 and above might find the below information useful.
RS5 (Build 17763, Windows 10 1809) update brings improved parity write performance to storage spaces. The improvement comes from being able to bypass the parity space write cache for full stripe writes. Previously created storage spaces will also benefit from these improvements (once the storage pool is upgraded with Update-StoragePool). For best results, you will need to create a new storage space with specific interleave size.
Upgrade your storage pool to the latest version.
Get-StoragePool <NameOfPool> | Update-StoragePool
Are you sure you want to perform this action?
This will upgrade the StoragePool "TestPool" to the latest version. This is an irreversible action.
[Y] Yes [A] Yes to All [N] No [L] No to All [S] Suspend [?] Help (default is "Y"):
Verify that your pool is at least at "Server 2019" version or later
Get-StoragePool | ? IsPrimordial -eq $false | ft FriendlyName,Version
NameOfPool Windows Server 2019
Create a new parity virtual disk, with an interleave size of 32KB, 3 columns. This maximizes your flexibility in adding capacity to your space, and ensures that the data stripe size is 64KB, which will match the NTFS allocation unit (cluster) size of 64KB that you will use in the next step. (If you use the Storage Spaces Control Panel UI to create the space, it will typically have an interleave size of 256KB and an NTFS cluster size of 4KB, which doesn't guarantee that all writes will be aligned to data stripe boundaries)
New-VirtualDisk -StoragePoolFriendlyName <NameOfPool> -ProvisioningType Thin -Interleave 32KB `
-FriendlyName FastParity -Size 1TB -ResiliencySettingName Parity -NumberOfColumns 3
Go to disk management, initialize the disk corresponding to the newly created virtual disk, and format it with NTFS (or REFS) filesystem with an allocation unit (cluster) size of 64KB.
Verify that copying large files to this volume is fast. Provided you are copying from a source that is different from any of the virtual disks in the storage pool, you should be able to achieve a write performance that is close to 2x the write performance of the slowest physical disk in your storage pool. With typical consumer SATA hard disks, if your source is sufficiently fast (e.g. internal SSD), you should be able to hit 200MB/sec for copying large files.
You can use the performance monitor (perfmon.exe) to verify that your new virtual disk has a high "Write Bypass %". When correctly configured, you should expect this value to be >99%. The Counter set name is "Storage Spaces Write Cache"