Since early July I’ve posted many of my practices (mostly in a 50m pool, a few in Lake Minnewaska) in this thread on the TI Discussion Forum. I’ve noted that one of my favorite tuneup sets is also a great test of one’s ability to increase speed efficiently. This in turn is one of the best predictors of your ability to pace a race effectively. There are two versions of this set.
Version 1: Maintain Stroke Count (i.e. Stroke Length) unchanged while descending your time (i.e. improving your pace).
Version 2: Maintain Tempo (i.e. Stroke Rate) unchanged while descending your time.  For this test set, you’ll need a Tempo Trainer.
You can do this set for any repeat distance, but it will be easier to keep track if you swim repeats of 50 or 100 yds or meters.

In doing both versions I found that I could make far more significant improvements in time while maintaining SPL than while maintaining Tempo. Here are examples taken directly from the practices I logged on the other thread.
Version 1
August 4
8 x 100 Descend, controlling stroke count.
I stayed consistent at 75 strokes. I was more relaxed, yet a bit faster, than on a similar set I did last week at 77 strokes. That time I descended from 1:59 to 1:42. Today, taking two fewer strokes, I descended from 1:53 to 1:38. I’m very pleased with being able to swim 15 seconds faster for 100m without adding any strokes.

Version 2
August 21
6 x 100 @ tempo of 1.10 seconds/stroke.. Try to reduce SPL progressively. (If I do, then speed ‘happens.’ I’m trying to be more efficient. Swimming faster is the result.)

I started at 85 strokes for 100m and finished at 80.
I felt exceptionally pleased to be able to subtract 5 strokes on this set. Yet that improved my 100m pace by only 5.5 seconds (I saved 5 strokes; each took 1.1 sec.. This is only 30 percent of the change I was able to effect two weeks earlier when I maintained SPL.

These two sets are really a mirror image of each other.
In Version 1, the task is to increase Stroke Rate, while keeping Stroke Length unchanged. (Note:When I did the set on Aug 4 I wasn’t trying to increase SR. I simply tried to ‘find’ more speed as artfully–not physically--as possible and the increase in SR just ‘happened.’)

In Version 2 the task is to increase Stroke Length while keeping Stroke Rate constant. (Note: In this instance I wasovertly and specifically trying to increase Stroke Length. Without highly conscious effort, it simply doesn’t happen.

What does this mean?
1) Stroke Rate is far more ‘plastic’ than Stroke Length. Which is more of a useful moment of clarity than a startling insight.
2) That you can easily increase Stroke Rate without trying. In fact, doing so accidentally, heedlessly and ineffectively is an almost universal ‘conditon’ of human swimmers when they try to go faster. But you can only increase Stroke Length with a conscious, and usually quite demanding effort.

I invite you to try both: How much can you descend without adding strokes?
Another day, repeat the same series, but keeping Tempo constant instead. How much can you descend now? Please share your results with us, and if you improve on further attempts let us know about it. If you take the test please post your results here.

I also suggest you Version 2 at different tempos. I did it today at 1.10. I’ll try again at 1.15, 1.20 and perhaps 1.25 and see whether a slower tempo allows a larger improvement in time.