mnu@RELENG_8_0:~> dd if=/dev/zero of=/tmp/gvirstor.dd bs=1m oseek=1999 count=1
1+0 records in
1+0 records out
1048576 bytes transferred in 0.003561 secs (294459461 bytes/sec)
mnu@RELENG_8_0:~> ll -sh /tmp/gvirstor.dd
1072 -rw-r--r--  1 mnu wheel   2,0G 19 тра 12:34 /tmp/gvirstor.dd
mnu@RELENG_8_0:~> s mdconfig -a -f !$ -u 12
s mdconfig -a -f /tmp/gvirstor.dd -u 12
mnu@RELENG_8_0:~> s gvirstor label -v -s 4Tb virtest /dev/md12
Total virtual chunks: 1048576 (4 MB each), 4194304 MB total virtual size.
Clearing metadata on /dev/md12.
Writing allocation table to /dev/md12... (8 MB, 2 chunks)
Storing metadata on /dev/md12 (500 chunks) (4 reserved) Done.
mnu@RELENG_8_0:~> diskinfo -v /dev/virstor/virtest
/dev/virstor/virtest
        512             # sectorsize
        4398046511104   # mediasize in bytes (4.0T)
        8589934592      # mediasize in sectors

mnu@RELENG_8_0:~> s newfs -O2 !$
s newfs -O2 /dev/virstor/virtest
/dev/virstor/virtest: 4194304.0MB (8589934592 sectors) block size 16384, fragment size 2048
        using 22825 cylinder groups of 183.77MB, 11761 blks, 23552 inodes.
super-block backups (for fsck -b #) at:
160, 376512, 752864, 1129216, 1505568, 1881920, 2258272, 2634624, 3010976, 3387328, 3763680, 4140032, 4516384, 4892736, 5269088, 5645440, 6021792, 6398144,
6774496, 7150848, 7527200, 7903552, 8279904, 8656256, 9032608, 9408960, 9785312, 10161664, 10538016, 10914368, 11290720, 11667072, 12043424, 12419776,
.....
etc etc

As for ZFS slowdown, try to download some gigabytes of torrents in parallel to it, especially when zpool is nearly full.