I have noticed that most apps that run on Mac AND Windows tend to be resource hungry and slower.
This is usual the result of software companies prioritizing the development of new features over software quality, and is has little to do with supporting multiple platforms.
I don't see it that way, because I hate it when software developers get lazy and take the easy way at the expense of the end-user.
Don't be quick to criticize the Lightroom developers, you are basing your opinion on two invalid assumptions.
Tasks requiring heavy computation (such as image processing) will be significantly slower when using an interpreted language, but interpreted languages are more than fast enough for many applications. I just wrote two simple programs to count up from zero for one second. The program written in a compiled language was about five times faster than the program that was interpreted, but the interpreted program was still able to count past 400,000. By comparison, the number of instructions required to process simple events such as mouse clicks and keystrokes is small enough that your brain can't perceive the difference. Most likely, the difference is probably smaller than the your monitor refresh rate.
Your second argument, that bad performance is the result of extra "layers" of code, is based on two faulty assumptions. First, it is incorrect to even assume that a cross platform app has more layers of code than a program written for one operating system. There are many approaches to cross platform development, one approach is to separate a programs code into a section that is platform independent, and to maintain two separate sections for handling windows and mac specific code. This code will operate at the same "layer" as any pure Windows or Mac application. It is also wrong to assume that a native application is not built upon multiple layers. Most OSX applications are written using one of two programming API's, Cocoa and Carbon. Cocoa and Carbon are software "layers" which are built on top of low level OSX components, such as Quartz. Some cross platform applications might use an extra layer on top of Cocoa or Carbon, but the performance impact will still be negligible. It is even possible to completely bypass Cocoa and Carbon and develop an application at a lower level than some off apples on software products.
I do share your opinion that Lightroom is inexcusably slow, but I was happy to discover that 40% of Lightroom's code was written in Lua. The use of interpreted languages can greatly increase programmer productivity and reduce the number of bugs, in the long term this is a good thing for users.
Lightroom's performance problems are mostly the result of poor memory management. I have noticed that Lightroom's memory usage will unexpectedly balloon for no apparent reason. (If it means anything to you, I suspect the culprit may be a poorly designed garbage collection system.)