One thing you may be seeing is a side-effect of how flash works. Flash emultating a disk can be quite lossy depending on how it's done. Most flash devices have an internal bitset that tells them which cells contain data and which don't. If you write to a cell that doesn't have data, it just does a write. If you write to one that's marks as having data (even if that data is no long important, from a deleted file, etc), if you're not writing out the whole block the flash manager does a read, merge, write, which takes more time.

That shouldn't affect swap performance (assuming the swap block size matches the flash block size, and the partition is "formatted" to align properly). But even something like writing files to flash is much faster on a clean device vs one that's had data on it and had it deleted.

A nice little app would be one that finds the "empty space" on your flash memory and resets those bits in the flash controller. But that's usually something you need a special API to do from the chipset manufacturer. (Intel Gen2 solid state drives, for example, have such an interface.)