Note you could also do VScript directly in map logic for convenience and cleanliness.
You could add these outputs to the flag entity to achieve the same results without having to set-up a trigger system:
Output: OnPickup1
Target: !activator
Input: RunScriptCode
Parameter: self.AddCustomAttribute(`move speed penalty`, [Move speed multiplier here], -1)
Output: OnCapture1
Target: !activator
Input: RunScriptCode
Parameter: self.RemoveCustomAttribute(`move speed penalty`)
Output: OnDrop1
Target: !activator
Input: RunScriptCode
Parameter: self.RemoveCustomAttribute(`move speed penalty`)
Note the backticks (`) are used instead of quotes since quotes would corrupt your map.