Speed isn't the only consideration. If you're using disk encryption, the only way to remove keys from RAM is to power off or hibernate. Preserving state between sessions with hibernation is much more convenient. Also, Linux only uses RAM * 2/5 as the size of the hibernation image [1]. You can make this even smaller by changing /sys/power/image_size. So with 16GB of RAM, it only has to write/read about 6.4GB.
You can use LVM to make the swap partition inside the encrypted container. If you don't want to use LVM, you can just use a swap file on an encrypted partition (but this isn't supported with btrfs).
