For those who weren't around when the threshold slider was a major talking points for developers corner (or whatever it was back then) I want to try to explain the slider as simply as possible.
The above is an example of several normal distributions. What the developers did was create maddens statistics for players with a normal distribution. Most players SPD (it doesn't effect acceleration which is how fast you reach top speed) hover around a mean value of 85-88 (don't remember the exact number but will pretend it is 88). This distribution is similar to the red line above when threshold is 50 and all player values are true values.
As slider threshold is reduced to 0 the line flattens to look like the yellow line. How this is done is the program compares the players statistic to the mean then then applies a bonus or penalty to reorder that statistic to the new distribution. The slower you are the slower you get and the faster you are the faster you get.
As slider threshold approaches 100 the line become narrower and taller as more values become clustered around the mean. The program in this case subtracts from faster players or adds to slower players based on their deviation from the mean.
For simplistic purposes of discussion we will pretend that a slider value of 0 or 100 alters values by 0.5 X the difference between the player value and the mean.
AT THRESHOLD 50 (REAL TEAM VALUES)
QB SPEED: 83, ACC 87
HB SPD: 88, 90 ACC 90, 91
WR SPD: 86, 96, 87, ACC 87, 97, 85
TE SPD: 73,86, ACC 83, 86
OLB SPD: 76, 88 ACC 85, 94
CB SPD: 93,92,91, ACC 94,87,92
Safety SPD: 91,86, ACC 87,83
AT THRESHOLD 0:
1. HB vs OLB: 88 is at mean so no change, OLB (76-88) X 0.5+76 = 70
Result: The linebacker was 12 slower now he is 18 slower a big difference
2. TE vs OLB: OLB new speed is 70, TE SPD is now 66
Result: The OLB was 3 faster than the TE now he is 6 faster not a big change
3. WR vs CB: WR new speed is 85, CB new speed is 96 the difference went from 7 to 11 making it harder for the WR to get open.
AT THRESHOLD 100:
1. HB vs OLB: 88 for HB so no change, OLB (76-88) X 0.5 reduced towards 88 equals 81. There was a 12 point difference now there is only 7 so HB has trouble with separation.
2. TE vs OLB: OLB new speed is 81, TE SPD is now 81 (technically 80.5) this makes them identical and now the TE route running may result in easier getting open.
3. WR vs CB: WR new speed is 87 so not much change, the CB speed is now 90 making them much closer and route may have more of an effect.
SUMMARY:
The reason I posted this is so everyone can understand why they see what they see. Sometimes in Madden you have a matchup that is improved by Threshold (favoring a fast player) and sometimes it is worsened or makes no difference depending on the matchup. You can't say Threshold improves WR being open or improves coverage since they are both true depending on the players involved.
Generally a threshold that is lower will make players who are elite speed look better, but because CBs are generally faster than all but a few receivers it will actually make coverage tighter resulting in the CPU struggle more on deep balls.
The changes above are compounded if you use All Madden level which applies CPU bonuses and player penalties to all stats including speed. This causes even more unpredictable results.
Consider this factor when you adjust threshold, and you are probably better off avoiding extremes since its effects are so variable.