DB update taking long time

I have a requirement to store the content of proto to mongoDB for which I am converting proto to json string and then converting to Document object.

Some time the proto will have so much data upto 14mb which makes the above conversion takes around 5 seconds, which is causing too much of latency in the code. Is their a better way to handle this ?

MongoDB Bson has the limit of 16 mb but this conversion comes with latency.

 >  private static Document serialize(AggregationProto aggregationProto) {
    try {
      String json = JsonFormat.printer().print(aggregationProto);
      return Document.parse(json);
    } catch (InvalidProtocolBufferException e) {
      throw new IllegalStateException("Failed to print aggregationProto as JSON", e);

Above is the method used to convert from proto to MongoDB Document.

Overall I am looking for something like below:

(proto <=> bson instead of proto <=> json <=> bson)

Reference: protocol buffers - Straightforward way to store protobuf java object in Mongodb? - Stack Overflow

Anything updated after this: BSON <=> google::protobuf::Message · Issue #2601 · protocolbuffers/protobuf · GitHub

DB update takes 2 sec and serialization takes 2 sec = total 4 sec.
How can this be improved?