MultiZarrToZarr speed improvements

Opening an issue so we can discuss processing time for MultiZarrToZarr. I'm running with the following code:

mzz = MultiZarrToZarr(
    json_list,
    remote_protocol='az',
    remote_options={
       'account_name' : 'goeseuwest'
    },    
    xarray_open_kwargs={
        'decode_cf' : False,
        'mask_and_scale' : False,
        'decode_times' : False,
        'use_cftime' : False,
        'decode_coords' : False,
    },
    xarray_concat_args={
        "data_vars": "minimal",
        "coords": "minimal",
        "compat": "override",
        "join": "override",
        "combine_attrs": "override",
        "dim": "t"

    }
)

mzz.translate('combined.json')

Running mzz.translate() takes upwards of 40 minutes on a list of 144 reference jsons, which were generated with inline_threshold=300. It looks like a good chunk of the time is divided between to_zarr() and split().

I've attached the profile file here as well, which can be visualzed with snakeviz ./multizarr_profile.

multizarr_profile.zip

Start with MultiZarrToZarr.translate() and profile the to_zarr() and split() paths using the attached multizarr_profile file and snakeviz. Compare the costs for 144 reference JSONs generated with inline_threshold=300, then verify that processing time is reduced without changing the combined.json output.