Quite likely. It's the same for 3D games on the PC, lower resolutions can maintain higher framerates due to having fewer pixels to render. Higher resolutions require more powerful graphics cards to maintain higher framerates.

The biggest fluidity-killer is background processes (almost always Maemo system processes) that grab a chunk of the CPU to perform some action. Modest is very, very bad at this, especially when responding to a mail notification with a couple things are going on. This will be somewhat resolved once multi-core chips are more common, however.