reads dont complete with parallelism

i have a collection with ~2000 documents, each of ~10k-20k bytes.. so maybe 10-40 megabytes.

when i submit a single get() of all documents in this collection via the node.js SDK, i get great response times. 11.54 seconds.

however, when i submit 4 at nearly the same time such that they run in parallel, i see that the reads finish such that they are barely faster than if i had submitted them sequentially. ~44 seconds.

i would expect to see, slightly slower in the multiple concurrent get() case, but not this linearly increasing behavior.

- OS: mac os x
- Node.js version: 20.11.1
- `@google-cloud/firestore` version: 7.10.0

#### Steps to reproduce

create a firestore collection of sufficient size and documents. execute a get() on that collection, see that it completes fine as a single request. then submit with multiple concurrent get() read operations, and notice that the time is almost scaling linearly.