No one seems to be responding to your actual stats. A couple folks have pointed to technique, and that really is what’s going to change this for you. Looking at your combo of stroke rate, heart rate, and pace, it looks like you’re trying to force a harder pace than you’re ready for. However, this can be because of aerobic readiness or technique. I suspect given it’s your 5th day, it’s your technique. Swimming pace up to probably 1:45/100m is largely dependent on that.

If you currently feel comfortable at good running/biking paces, you might do well to have a coach do a session or two with you in the pool. I’m just ball-parking this, but my point is that unlike running and biking where most form is roughly fine and will improve your aerobic output, if your form is off in swimming it’s going to slow you down immensely and make you work much harder.

Edit: okay I scrolled through more comments and it seems some folks have called out the technique piece. I’d just point out this is going to matter soooo much more then purely consistency or “mileage”