ZFS send much slower than before

With the exact same devices (computer, external hard drive…), my ZFS backup (via zfs send | zfs recv) takes much longer than before. Now, it takes around 1.5 to 2.2mn to copy 1Go, but before I think to remember that it could easily copy multiple Go per mn (with around 100M/s if my memory is correct, so maybe 6G per mn if it's possible).

$ zpool list
NAME                SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
zfs_pool           1.63T  1.22T   420G        -         -    47%    74%  1.00x    ONLINE  -
zpool_my_passport  1.81T  1.68T   135G        -         -    17%    92%  1.00x    ONLINE  -

(passport is the external hard drive (it is NOT SSD, but zfs_pool is an SSD)) I feel like the issue started when the external drive started to be full (I now need to remove old snapshots), but 17% of fragmentation does not seem that bad (but 47% seems worse no?). Any idea if fragmentation could be the issue here, and if I can somehow restore the old performances?