gistfile1.rb

#https://github.com/spree/spree/blob/master/core/app/models/spree/stock/coordinator.rb#L32:L40

def build_packages(packages = Array.new)
  StockLocation.active.each do |stock_location|
    next unless stock_location.stock_items.where(:variant_id => inventory_units.map(&:variant_id).uniq).exists?

    packer = build_packer(stock_location, inventory_units)
    packages += packer.packages
  end
  packages
end
      
# refactored to
      
def build_packages(packages = Array.new)
    # OR StockLocation.for_variant_ids(inventory_units.map(&:variant_id).uniq)
    StockLocation.active.joins(:stock_items).merge(StockItems.where(:variant_id => inventory_units.map(&:variant_id).uniq).each do |stock_location|
      packer = build_packer(stock_location, inventory_units)
      packages += packer.packages
    end
    packages
end

To summarize and tidy up this would be what we'd need:

class Spree::StockLocation
  scope :for_variant_ids, ->(variant_ids) { active.joins(:stock_items).merge(StockItems.where(variant_id: variant_ids) }
end

class Spree::Stock::Coordinator
  def build_packages(packages = Array.new)
    Spree::StockLocation.for_variant_ids(inventory_units.map(&:variant_id).uniq).each do |stock_location|
      packer = build_packer(stock_location, inventory_units)
      packages += packer.packages
    end
    packages
  end
end

Do you want me to do this directly in our customized version of Spree or do it against master and open a PR then backport to ours?

Yes mate, add to core please