The problem with Windows "apps" is that they are made inefficient and un-optimized by default, because they were targeted at much more powerful processors, and as processors became even more powerful, they didn't care about increasing the performance or at least keeping it at the previous level (and not slower).

Developers never really cared about netbooks, and these devices, either with ARM processors or with Atom, are only at that level. So most Windows apps will struggle on these machines, while most apps on iOS or Android will be a lot faster on the same processors, because their low-end target was something like a 600 Mhz ARM11 CPU.