This is an interesting set. One comment is that the stroke rate seems low.
1.3 to me is more a drilling speed (and I'm not fast). The stroke count is impressively low (I'm not clear whether the pool is yards or metres but it's good either way) but it doesn't surprise me that you have to put in a lot of effort to achieve the higher speeds. A slightly higher tempo would give more a more continuous stroke and so be easier even if the SPL is higher, which is consistent with your experience with 18SPL.