ReadyBoost improves things by caching boot files on media with fast access time. It's much like ureadahead but using an agile medium instead of just stacking the files on the boot disk. Whether or not it actually works better than something like ureadahead is highly debateable.

A Flash drive might have a lower seek time than a mechanical disk but throughputs are often very limited (there are exceptions of course: USB3 SSDs, for example). It's possible that by removing the seek element (as it does by stacking the files) ureadahead is faster because the drive has the faster throughput.

ReadyBoost has no effect on after-boot performance, IIRC, though it's concievable they might put some memory paging on there too if it's fast. You can put your swap on an external drive but that's only viable if the disk is faster, in both access and throughput.

In answer to your question: plugging a slow mechanical disk into a slow USB2 bus will give you no benefit other than extra disk space. Even if ureadahead could use it (which I don't think it can) any benefit would rely on the external drive having a higher throughput than your internal disk.

If that's the case (your internal disk is slow) you really want to consider upgrading it. Your primary disk should be the fastest permanent storage in your system. If you're considering an external swap, have a look at the prices of RAM. Stupidly high quantities are very cheap these days.