### Description

![image](https://github.com/elastic/elasticsearch/assets/76245/d0443170-0419-4759-b505-7620ff3d6594)

The execution time of grouping stats by multiple keywords has increased: 1.2 to 1.6 seconds (with doc_partitioning), 4.3 to 6.7 seconds (with shard_partitioning). The equivalent query with the _search API takes 1.8 seconds.

`from nyc_taxis | ... | stats c = count(dropoff_datetime) by trip_type, payment_type, vendor_id`

This regression occurred since July 2, 2023, and I suspect it may be due to the PR: [Support multivalued fields on arbitrary grouping](https://github.com/elastic/elasticsearch-internal/pull/1340).

/cc @nik9000
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/blockhash/PackedValuesBlockHash.java
}
                Arrays.fill(bytes.bytes(), 0, nullTrackingBytes, (byte) 0);
                bytes.setLength(nullTrackingBytes);
                if (singleEntry) {
                    addSingleEntry();
                } else {
                    addMultipleEntries();
                }
            }
            emitOrds();
        }
        private void addSingleEntry() {
            for (int g = 0; g < groups.length; g++) {
                Group group = groups[g];
                BytesRef v = group.encoder.read(group.valueOffset++, scratch);
                if (v.length == 0) {
                    int nullByte = g / 8;
                    int nullShift = g % 8;
                    bytes.bytes()[nullByte] |= (byte) (1 << nullShift);
                } else {
                    bytes.append(v);
                }
            }
            int ord = Math.toIntExact(hashOrdToGroup(bytesRefHash.add(bytes.get())));
            ords.appendInt(ord);
            addedValue(position);
        }
        private void addMultipleEntries() {
            ords.beginPositionEntry();
            int g = 0;