Decaying slow vs rapidly decaying slow?

I've always assumed a decaying slow reached half its value over half the duration. For example, lv 1 Lulu's Glitterlance would slow by 80% initially, 60% after .25 seconds, 40% after .5 seconds etc

What about rapidly decaying slows? Is that an exponential decrease over time? A link to any existing articles on the math would be appreciated.