Why chrono::timezone and format are slower than localtime_s and stringstream?

There are two main reasons why the "new" version is significantly slower than the "old" one:

• std::chrono::currentzone()->tolocal(now).

This performs a full IANA/Win32 time zone lookup and applies all historical DST transitions. On Windows, it calls Win32 Time Zone API under the hood; on POSIX, it reads TZDB files. Even if the data is cached after the first lookup, each call still binary-searches in-memory tables and applies offset calculations. This is modern solution that avoids many problems old one had. By contrast, localtimes(&loc, &tc) is just a thin wrapper over the CRT's process-wide tzinfo table, which is simpler and much faster to work with.

• std::format("{0:%F} {0:%R}:{0:%S}", /…/);

std::format uses a generic formatting engine that dispatches through std::formatter specializations for dates, performing locale checks, width and fill handling, and so on. Even though the format string is parsed at compile-time, the runtime still must traverse formatting tables and apply all formatting rules. So, even known compilers optimize a lot, std::puttime(&loc, "%F %T.") is still faster, because it is time-specific formatting wrapping over strftime, which is plain C and optimized heavily for date and time formats.

• Avoiding std::stringstream entirely. It can be replaced with std::formatto or fmt::memorybuffer. For maximal performance, consider snprintf into a fixed buffer directly;
• Caching tzinfo and statically allocating char buffers to avoid dynamic allocation at all.