Most efficient sorting on nested values

[scarabas (Scar)]

I am sorting results from Elasticsearch (5.1.1) based on a calculation of values in nested key/value structures.

The sorting has to:

1. find values from given keys across multiple nested structures
2. multiply those values with one another
3. use this multiplication as a score for sorting

What I have currently is working, but it's really slow/inefficient. I have made a Painless script, due to the calculations in #2 above. What I am doing is:

1. loop through all keys to find their respective matching value
2. for the first match, save the value in a variable; for subsequent matches, multiply saved value with current value, and save that in the aforementioned variable

I think the inefficiency is due to:

1. looping though all nested items (there are a lot per document, and many documents)
2. I am using params['_source'], which has a reputation of slowing down things. AFAIK, I have to use params['_source'] to address nested values in Painless

Now to the question: how can I solve this problem more efficiently? Am I going about this the wrong way entirely, or is there a way to not use params['_source']?

My mapping (the nested structure is "my_ratios"):

{
"my_index": {
"mappings": {
"my_type": {
"properties": {
"a_value": {
"type": "long"
},
"my_ratios": {
"type": "nested",
"properties": {
"Key": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 256
}
}
},
"Value": {
"type": "float"
}
}
}
}
}
}
}
}

Example of nested key/value structure:
{
{
"Key": "Key1",
"Value": 0.4898
},
{
"Key": "Key2",
"Value": 0.14286
},
{
"Key": "Key3",
"Value": 6.12245
},
...
}