It's already on there for some of the benchmarks at least. It generally is roughly between 30th and 60th place depending on the benchmark (with outliers in both directions). That said, HTTP.jl is not a library that has had a ton of work in Julia, and it's probably not what you want to write a high performance webserver in at the moment. I don't think this is a fundamental limitation, but just a domain that needs a couple thousand dev hours to make competitive with some of the more mature frameworks. Another part of the problem is that Julia has some fairly well known garbage collection performance problems. GC performance hasn't been a major dev priority as of yet, since the language is much better than most high level languages at eliding allocations or stack allocating variables (so it doesn't make as much garbage for many workflows). That said, for GC heavy tasks (string stuff often is), it is a major problem that is starting to get a bunch of effort put into it. Hopefully in a year or 2, these issues will be solved. It's nothing too fundamental, just that the current GC only has 2 generations, and the heuristics for moving objects between generations are not tuned especially well.
Is there any effort to include Julia in the TechEmpower Benchmarks? If it's trying to be a general-purpose language, that's the first place I'd look.