JIT is not an interpreter! Is a Just-In-Time compiler. Also .Net is a platform (which includes a VM, that is in case of .Net a JIT, and in case of Compact Framework an interpreter). The JIT generated code is as hard (or as easy) as any C++ code, and the base code is as hard to decrypt (if you mean the the .Net's bytecode that defines the programs, there are many obfuscators to make sure that no one can read them).
When we talk both about .Net and about Mono, because they are not interpreters, they can generate output of the program which is fully compiled (as .so on Linux, and as native .dll on Windows) that is used to execute the code you are running.
There are many cases when .Net code works in the same performance range as it would be written in a native language, like Paint.Net/Pinta, sometimes they may work even faster (MonoDevelop/SharpDevelop in .Net/Mono world, or Eclipse/IntelliJ IDEA) than their opensource equivalents (Adjunta, DevC++), sometimes slower (in Java world, comes Netbeans to mind, even though Netbeans improved a lot the performance in the last releases).
In the list of languages, I'm really familiar with Pascal, and I can say that is not faster than C# on Linux. FreePascal is basically tied performance wise (even though memory wise Pascal seems to be the most efficient of all):
http://shootout.alioth.debian.org/u6...l&lang2=csharp Anyway, I'm thinking that statically compiled with LLVM may run faster on phones (like Mono for Android or for iOS) than the FreePascal equivalent. The reason is because Mono can be compiled into optimized code, and FreePascal has a more limited optimizer (is in the same class as the JIT of Mono, it performs a bit more optimizations, but it has no good registry allocator).
At the end, did you tried in your real applications to use Mono? To see if is fast(er)? Are you using just XSP and based on this you make up your mind?
As for my machine, I did precompile most of applications on Mono (search for AOT = ahead of time compilation), to reduce startup time, but performance wise otherwise is really unoticiable (I think it should be faster in JIT mode).
Do you have any numbers when Mono is slow to you?