fromHexString function has been optimized by directly converting characters to bytes using bitwise operations and manual parsing within the loop, eliminating the use of std::stoul and reducing the creation of temporary objects to enhance performance.

1. fromHexString and fromHexStringToUint64 assume that the input after removing "0x" is a valid hexadecimal string without any non-hexadecimal characters.
2. The original function creates a substring for each byte conversion (hex_str.substr(i, 2)), which involves both allocation and copying of two characters into a new string object for each byte of the input hex string. This is computationally costly, especially for long strings.
3. This avoids that by directly parsing the substring withstd::stoul,thusly saving time and memory by minimizing reallocations.

Also... error handling.

By reserving the required amount of space in the vector upfront (bytes.reserve(cleaned_hex.length() / 2)), the function avoids the overhead associated with the vector's automatic resizing during the push_back operations.

Normally, if you don’t reserve space, the vector may need to resize and copy its contents multiple times as it grows, depending on the implementation. Each resizing involves allocating new memory and copying the existing elements to the new space, which is computationally expensive. This significantly improves performance primarily by reducing dynamic memory allocations and eliminating redundant operations