Streamline position and fatigue
Novice swimming here. I've found that I've been getting faster the more "straight" I keep myself with my core tight, butt clenched, and trying to keep an upright out chest in the water. I'm not that fast, but my average time dropped by 10-15 seconds/100yds from 2min/100yds ish when I'm "streamlined". However, I can't maintain the form too long as I get so tired. My shoulders are naturally hunched and rounded from deskwork all the time as well as too much chest workouts so it feels tiring to keep my shoulders back too. I usually do front quadrant timing for my strokes.
I can only hold it for 50 or 100yds, but after that it's so much easier to just go somewhat limp. I can swim forever if I just let my butt, core and chest slump, I'm just super slow like 20 seconds/100 yd slower.
Does anyone else feel tired trying to maintain their "form"? Does it get better? The only thing I can think of is that I'm trying to be too tense while keeping "straight, but I become slower when I relax too much or maybe I need to push myself and try to go farther.