Might not need to decomplie all assemblies. The Code Search add-in for .NET reflector 188.8.131.52 which I currently use, works this way: when you select an assembly, then run the Code Search, after you type some words to search, it searches through the whole assembly to find all matches in the code, no need to search other assemblies other than the selected assembly itself. When you select a namespace, type or specific method in that particular assembly, it search through the selected namespace, type or the selected method. So the performance isn't that bad, but the functionality it provides is really useful and powerful.
Another thing is, in the disassembled code window it'd be also very handy if it can provide ctrl+F to search the code window as well :)