Is it necessary to encrypt a swap * file * if it will reside on an encrypted root filesytem

Indeed, the page describes setting up a partition, but it's similar for a swapfile:

dd if=/dev/urandom of=swapfile.crypt bs=1M count=64
loop=$(losetup -f)
losetup ${loop} swapfile.crypt
cryptsetup open --type plain --key-file /dev/urandom ${loop} swapfile
mkswap /dev/mapper/swapfile
swapon /dev/mapper/swapfile

The result:

# swapon -s
Filename                                Type            Size    Used    Priority
/dev/mapper/swap0                       partition       4000176 0       -1
/dev/mapper/swap1                       partition       2000084 0       -2
/dev/mapper/swapfile                    partition       65528   0       -3

swap0 and swap1 are real partitions.

Getting swap back to 0 is not a useful goal.

There is nothing ipso facto wrong about having things in swap. It is quite possible for a program to load resources it doesn't actually use, and for the kernel to notice this and swap them out, freeing the memory for use by programs that can actually make use of it right now. This situation comes up a lot in today's modern bloatware world, where programs depend so much on huge libraries others provide, even though they need only a tiny fraction of the library's full capability.

The only hard numbers you have provided — 200 GB in 30 minutes — also looks quite good to me. That's 114 MByte/sec, which is an impressive copy rate, considering that you're copying a file within a single physical volume. It wasn't that long ago that 100 MByte/sec on purely sequential reads was pretty impressive. You're managing better than that with interleaved reads and writes!

Bottom line, I think you're barking up the wrong tree.