That is true, but it is of course slower. And the goal of this threat is faster not better compression.
I/O is not the bottleneck of xz on higher presets, your CPU is it. And it might even be slower, because the filesystem is compressed, too.
The same computer, but with little more going on. So it is not really comparable to the other test.
Option -6 same tar file in RAM 1:29.22 and on my SSD 1:31.80 and again HDD 1:32.70. I would say it is nearly the same.
If you build aur packages, first try building in RAM. Normally you can use your /tmp, which is mounted with tmpfs.
If you only want to install these packages and don’t want to save or use these packages on other computer, don’t compress at all.
@jonathon mentioned a way in this post.
When space is the issue and you have time, compress with higher compression presets. But keep the memory use in mind. With -6 and T 4, on my computer xz uses around 550 MB of RAM.