If it's specific apps, usually it is just bad coding. 'Enterprise Java' style coding where performance is not even tested for, let alone designed into the algorithms.

Usually culprits are things like downloading multiple data files in a single threaded block, or insanely deep object graphs with thousands of memory fetches per real operation.

That's not really anything to do with the technique (code in browsers) or the runtime (JVM) or even the programming language (Java) -- and everything to do with poor development.