Why is C slower than Bolin?

There are always going to be tradeoffs -- as your article admits, the differences themselves come down to a 1 cycle difference, which would show up as about 1ms. If performance is really critical for an application, things like this are more than capable of being rewritten in an optimized form rather than relying on the standard library, which understandably doesn't try to cater to every use case. Or in other words, when performance is a concern, C is more than capable of providing the necessary tools to achieve it.

I don't think a comparison like this will convert anyone from C to Bolin. Where a possible argument may be made is that Bolin is a better language, and argue for the design decisions that were made in its implementation. Even so, people are bound to disagree, and the bottom line is that while C is imperfect, that's what makes it perfect.

C does not enforce rules on how its authors thought code ought to be written, and neither does C enforce one type of memory management over another. C has no preference over the way packages are managed, and neither does C prefer one standard library over another. C is unopinionated: it lets you do whatever you want to do however you want to do it.

Is there an easy way to implement tree structures (as needed for example in a compiler)?

PS: anyone who is interested should definitely click the download link and read the EULA...