A best time is a very narrow measure
A maximal time summarizes what happened over one distance on one day. Fatigue, start quality, pool conditions, lane traffic, and pacing can all affect it.
Keep the time, but add other markers that describe everyday training more clearly.
Look at consistency
On a repeated set, compare the spread between repeats. If your 100s cluster more closely, you are controlling pace better. Also notice whether the final quarter of the set still looks technically similar to the first.
Being able to reproduce a level of swimming is an important kind of progress, especially for endurance.
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Same time, lower cost
You might swim 100 m in 1:45 today just as you did a month ago, but with calmer breathing, fewer strokes, or faster recovery at the wall. The time is unchanged; efficiency or aerobic capacity may not be.
Conversely, a faster repeat achieved with completely disorganized swimming may not be the progress you were looking for.
Notice what survives fatigue
A technical cue that is easy during warm-up becomes more meaningful when it survives the main set. Maybe your head stays quieter, breathing disturbs alignment less, or your turns remain clean late in the workout.
Those signs are hard to reduce to one number, but they directly change the quality of your swimming.
Use a few markers, not twenty metrics
Pick two or three observations tied to your goal. For example: average time over 8 × 100 m, spread between fastest and slowest, and perceived effort. Or stroke count over 50 m, breathing comfort, and turn quality.
The goal is not to measure every length. It is to leave enough evidence to recognize improvement before it eventually appears as a faster clock time.