NumberParser is used in BSONElement, the SBE VM, and many _gen files. This class uses a helper function parseNumberFromStringHelper, where I see two unnecessary string copies: #1 and #2.

In an extreme case I came up with, parsing one million numbers from {$match: {a: 1, b: 1, c: 1, ...}} originally took 4 seconds, and with a few lines change to avoid a copy it took 2 seconds instead, and the patch was green.

We should also look at the Decimal128 constructors, they appear to make unnecessary copies as well.