Julia is suited for long running processes, be it a simulation/calculation that will run millions of times for hours or a long interactive session for example exploring multiple variants of an algorithm (such as using the REPL, Jupyter or a combination of REPL and text editor which is my favorite, thanks to tools like Revise.jl which will automatically compile code as soon as you save the file).
The start will be slow, since the compiler will be aggressively optimizing and compiling everything it can (not unlike if you had just the source code of a C++ program and wanted to run for the first time), but over time pretty much everything will be already compiled to high performance code so the program, if written adequately, will run at similar speed to C, but it can still be modified at runtime like any dynamic language (which it can then compile as well).
The start will be slow, since the compiler will be aggressively optimizing and compiling everything it can (not unlike if you had just the source code of a C++ program and wanted to run for the first time), but over time pretty much everything will be already compiled to high performance code so the program, if written adequately, will run at similar speed to C, but it can still be modified at runtime like any dynamic language (which it can then compile as well).