When the battery was crapping out on my Zero DS 6.5, the difference in range was most noticeable at the higher speeds. At very slow speeds, such as on a 40 mile unpaved road, I didn't really notice any difference in range, but I think it was reduced a bit there also, but not nearly as obvious as at higher speeds.
What was also very obvious is my charge times was much LESS. Just as if my 6.5 KWH battery was a good 3 KW battery. It took less than half the time of normal to get to 100% charged. Also, regen would appear to charge the battery much better, when looking at the SOC, which probably helps explain the good range at very slow speeds. Regen would add as much range as ever, as the SOC increases were double and very noticeable as I was going down hill at slow speeds.
Another symptom was if I got back at 5% SOC and had to slow down a lot for the last mile (1.5 km) or so, the SOC would start to increase to perhaps 15% soc or even more if I just let the bike sit.
Now it has a new 7.2 KHW battery (replaced under warranty) and what a difference! The difference is most noticeable on the freeway, where my range is now several times what it was with my old battery.
The battery crapping out during the warranty is good, especially if you're getting a free upgrade as I did. My DS ZF 6.5 is now really a ZF 7.2!
-Don- Reno, NV