There's an initial spike implementation of this in https://github.com/DamianEdwards/RazorSlices/tree/ReusableSlices

To use, simply add @implements IRazorReusableSlice to the top of your slice .cshtml file. You can optionally implement the bool TryReset() method if your slice has instance-level state it needs to clear before reuse.

Some initial results from adding support for this:

This image of the Visual Studio profiler (.NET Object Allocation Tracking) shows that (after warmup) the only thing allocated when rendering a reusable slice to a string 1000 times is the final string itself:

Here's benchmark results comparing rendering the same template via a regular slice vs. a reusable slice: