Due to the fact that there was no reliable source for this information anywhere, I recently went to the trouble (well not really trouble, it was actually quite fun) of testing different times on every level until I discovered all of the times required for the so-called speed secrets. Aside from easy, most of the information you will find elsewhere is WAY off (people seem to either just post their times or pull random numbers out of their ass) and you don't have to do the normal/hard levels nearly as fast as you probably think. Hell for hard it's pretty difficult NOT to unlock the secrets if you can actually beat the levels and don't get lost.
The time given is the time that must be beaten in order to unlock the secret; a tie will not count (yes I actually tested that, it was kind of annoying). The thing in parenthesis after the time is what you unlock obviously.
Tomb: 1 min (Paintball cheat) Chinese: 1 min 15 s (Chinese Waiter) Cyberden: 50 s (Siamese Cyborg) Village: 30 s (All Heads Detachable cheat) Chemical Plant: 30 s (Male SWAT and Female SWAT) Planet-X: 1 min (Green Alien) Mansion: 3 min (Big Heads cheat) Docks: 1 min 15 s (Male Solider and Female Soldier) Spaceways: 1 min 5 s (Red Alien)
Tomb: 4 min 30 s (Priestess) Chinese: 3 min 25 s (Gun Sounds cheat) Cyberden: 4 min 30 s (Tuxedo Cyborg) Village: 3 min (Hick Hyde) Chemical Plant: 1 min 30 s (Infinite Ammo cheat) Planet-X: 4 min 20 s (Float Alien) Mansion: 5 min 30 s (Overall Mutant) Docks: 3 min (Big Hands cheat) Spaceways: 1 min 20 s (Female Alien)
Tomb: 8 min (Teeth Mummy) Chinese: 6 min 20 s (Mr. Big) Cyberden: 7 min (Enemy Rockets cheat) Village: 4 min 45 s (Insect Mutant) Chemical Plant: 5 min 30 s (Malehood) Planet-X: 5 min 15 s (All Characters Headless cheat) Mansion: 15 min (Girl Zombie) Docks: 7 min (Shock Trooper) Spaceways: 6 min 30 s (Small Heads cheat)
Just figured I'd share this information here just in case anybody was looking for it. I have also posted it on the TimeSplitters Wiki as a page called "Timed Unlocks."