When using a negative potion effect value applied by a command block through the effect command, it is not applied.
Example:

/effect @p 2 30 -6

Expected: The player is given a slowness effect but actually moves faster.
Result: No change.
Test 2:

/effect @p 1 30 -6

Expected: The player is given a swiftness effect but actually moves slower.
Result: No change.