I'm mildly depressed about the fact that there is no wiki article to be found after the migration that explains this properly...
Simply spoken, do the following:
1. Scroll to the end of the slider
2. Open the timing menu (F6)
3. Press the green + here and follow up with "OK" to close the menu
4. Scroll to the start of the slider
5. Open the timing menu (F6)
6. Press the green + again but this time don't go "ok" right away. Instead adjust the value here:
A value lower than 1 will result in a slider that is slower than your other sliders.
Then confirm with "OK".
Step 1-3. is only done to not have your slider velocity change affect all following sliders as well. Always put a section at the end of your slowdown/speedup before doing the actual slider velocity change to avoid snapping errors.