Currently, in PlanCacheSizeParameter::parse in src/mongo/db/query/plan_cache_size_parameter.cpp, we take an std::string reference as input and use it to match against a regular expression. We also need to do a lot of std::string conversions on the output.

However, we don't really need std::string.  matchView will accept a StringData as input. Then the std::stod can be changed to exploit our knowledge of m[1]'s format.
We know it's digits, a dot, and then maybe more digits.

The parseUnitString function should just be changed to take StringData.

double size;
invariant(!std::from_chars<double>( 
    m[1].begin(), m[1].end(), size, 
    std::chars_format::fixed).ec);
auto statusWithUnit = parseUnitString(m[2]); 

We will use an invariant instead of uassert because it would indicate that our compiled-in regex was wrong, in that it didn't have the right captures to produce a valid fixed point number.