Yes, I've heard that before, but comments like this one in your linked issue still make me wonder:

> at least for gcc and Visual Studio using #pragma once has a significant impact. The fact is, the compiler does not need to continue parsing the whole file when reaching a #pragma once. otherwise the compiler always needs to do it even if the include guard afterwards will avoid double processing of the content afterwards.

As written the explanation for these optimizationst suggest that both "pragma once" and include guard optimization still requires opening and closing the file each time an include is encountered, even if you bail after parsing the first line. Is that overhead zero? Or are the optimizations explained poorly and is repeatedly opening/closing the file also avoided?

Either way, do you know what causes the slowdown as a result of including <meta>?