The Concurrency Kit works with Unity JS, but only the "fiber" and "coroutine" features are supported which won't help you here. JS doesn't support .NET threads which the rest of the kit is based on. JS uses web workers or threading.
That said, Unity is designed so that you can only interact with its objects on the main Unity thread. So the code you've written there can't be offloaded to another thread. I don't know why Unity didn't provide async methods for these kinds of long running operations.
Are you creating multiple sprites in a loop? If so, just yield between each iteration and you may not see the pause.
Depending on your scenario, you may also consider whether you can pre-load the sprites at scene setup when the user may already be waiting for the scene to load.
A related option would be to create a pool of sprites on scene load, and then reuse them throughout your scene playback, acquiring and releasing instances back to the pool as you need them. That way you may spend time updating a texture but not recreating the Sprite instance.
Creating objects in Unity is expensive, so try to pool as much as possible.