I haven't seen you swimming but first thing that comes to my mind when I hear people haven't difficulty maintaining speed above 75-100m is breathing issues. People can become good and get away with breathing partners issues or flaw under 100m like you mentioned 1:30 (90 secs) isn't that long but once you get over 2 mins than it bites you hard.. and your stroke will fall apart because you are trying to get some air in and probably pausing or doing anything possible that disrupt your technique for air.

Like others mentioned putting on more volume will help but 1:30/100y isn't fast and should be attainable simply with good technique no hard effort.

Get yourself film under water would love to see how much air you are blowing out... Are you inhaling/exhaling at the same time when grasping for air? Pretty common mistake by non-swimmers... ;)

Edit: BTW I feel the breathing is still a limiter for me even swimming in the 1:10-1:15/100y pace... I can improve on that technical item to get faster without having to pound in more mileage, disclaimer I have no swimming background what so ever... it's my pure feeling, I think it will help me be more efficient with other phase of my stroke.