The current BigDecimal(String) constructor calls String#toCharArray, which has a memory allocation.

public BigDecimal(String val) {
    this(val.toCharArray(), 0, val.length()); // allocate char[]
}
When the length is greater than 18, create a char[]

boolean isCompact = (len <= MAX_COMPACT_DIGITS); // 18
if (!isCompact) {
    // ...
} else {
    char[] coeff = new char[len]; // allocate char[]
    // ...
}

We can eliminate these two memory allocations to enhance performance.