The most famous of the "classic blunders" mentioned by Vizzini in the Princess Bride - "never get involved in a land war in Asia" - was reputedly advice given by General Douglas MacArthur to President John F. Kennedy, in 1961.
Yeah your game is running pretty fast already, its going to be hard to show a feelable speed improvement when you only have 5-10 second between turn times (believe it or not some players are waiting over 30 seconds between turns). In a game with only 5 players its going to be hard to see, try a game with 18 for a more significant difference.
But I did measure it out and I played out the next 5 turns with and without speed mod (all time in seconds):
Speed mod time / normal game time / percentage improvement 1. 5 / 6 / +17% 2. 6 / 7 / +15% 3. 6 / 8 / +25% 4. 7 / 9 / +23% 5. 5 / 6 / +17%
So its faster, but at your low times its hard to feel a change.