First congrats on finish your 1.2 mile swim in 48 min. Youíre roughly mid-pack for triathlon, nice!
Swimming at slower tempos is great for balance, but not necessarily in longer distances. If you feel comfort at 1.4+ (~42 spm) and rushed at 1.3 and below, there are probably pauses/hitches in your stroke and added arm/hand movements eating up time. If youíve been swimming a lot at 1.6, Iím certain you are pausing the hand at the hip and at entry which stunts the recovery and overall stroke rhythm.
Break your longer sets into 50ís (or 25 repeats) and clean up any pauses and unnecessary movements at slightly faster tempos. Do 10 x 50 descending from 1.3 - 1.2, i.e., 50 @ 1.29, 50 @ 1.28, ..., 50 @ 1.20. Shorter sets will allow you to stay focused on tempo and not feel so rushed. The .01 drop in tempo is subtle and allows an easier adaptation cleaning up hitches/pauses in your stroke.
Get comfy at faster tempo before moving on to longer sets. Your stroke starts and finishes at forward extension, its a fluid and continuous motion from start to finish, no pauses, hitches, or added hand movements (flipping, bending wrist).
Enjoy your journey!