This question has already been well addressed as far as defining when swap on RPi is usefull vs when it would be considered thrashing. Here's a quote from a memebr of the RaspberryPi forums "Heater" because they've stated it clearly;
A swap partition is not designed to enhance performance and if your program/data is constantly being swapped from RAM to swap space performance will slow by a factor of hundreds or thousands. This would be called "thrashing".
What the swap technique does is help run a lot of programs in limited RAM space reducing the possibility of an "out of memory" failure. This might be useful in a multi-user system or other server system. Imagine a hundred people logged in and editing some files, they are all working quite slowly and the overhead of swapping from one users data to another as needed may be quite acceptable.
In that way swap helps performance by not having things crash out when they run out of memory!
As has been said before I would not put a swap partition on an SD or USB stick because it is slow and for fear of wearing it out.
That being stated I'll direct my energies towords helping you gain a bit more speed and reliobilaty as well as some suggestions for things you may not have heard of.
Here's a guide on RAID setup on how to squeez a bit more performance out of the dirves that you plan on sacrificing as well as the drives used to store data you wish to keep
RAID on RPI with encryption support.
However some things to note about this guide that you'll want to modify;
leave off encryption if you don't need it; espechially for the swap partition as this would require more recources from the RPi.
don't use the swap USB RAID drives for anything else other than swap. Keep these devices marked some how so that they don't get used by some one else that may expect reliability.
- USB thumb drives and SD cards where not originally designed to handle the constant abuse that swap will put'em through. Keeping it off the SD card is a good start as elinux.org had this to say about the RPi's through-put on SD cards
maximum throughput of the card reader of the Raspberry Pi is 25 MB/s and that most likely read and write speed won't exceed 22 MB/s.
If moving your OS's folder structure to a USB RAID too then keep these RAID drives seperately marked to avoid mixing them up with swap used drives.
Speed performace with RAID on RPi (from the resurch I've already done) only seems to provide a boost to the first two drives. However most build use USB hubs to attach thier drives so hooking two drives per hub and the using multiple hubs directly attached to the RPi may give you even more of a boost... maybe... Or you could try hooking the USB drives directly to the RPi (the modle B has 4 ports) and cut the hub's interfence and speed constraints out of the equasion entirly.
USB read write speeds will be limited by the through-put of the slowest link in the trasfer chain; this includes any plasebo effect that you may have expereanced with 3.0 USB drives. So if using a USB hub then the max through put is the port that it is connected to as well as being limited by the amount of free resources your RPi has for trasfering files.
If you have a spicific task or directory structue to be accessable even faster consider chaining more than one RPi togeather in what is know as a bramble stack; hint search on the YouTubes for an author named TinkerNut, they've a quick two parter on the subject that is great! by using more than one RPi with each having the above sugested boosts your maximum through put would be more limited by the ethernet networking caps of the RPi and attached router.
If you wish to run these RPi for extened periods of time without need of replacing the drives consider solid state or the clasic hard drives for better relioblaty. And I would also advise making a cron job script that backs up improtant files that change to a compleatly seperate device; just to be safe.