“You gain a non-addictive subdermal Turbo (chem) injector. This perk may be taken twice, with the second rank increasing the effect from 2 to 3 seconds and the uses per day from 5 to 10.”— In-game description
Implant GRX is a non-addictive drug with the same effects as Turbo: slowing down your surroundings to give you more time to act. It is accessed from your inventory in the Aid section of your Pip-Boy, with the supply replenished every 24 in-game hours. This perk can be taken twice, for enhanced effects:
Rank 1: it can be used 5 times a day, and lasts 2 (pre-Turbo) seconds per dose.
Rank 2: it can be used 10 times a day, and lasts 3 (pre-Turbo) seconds per dose.
The stated duration is for time before it's been slowed by the Turbo effect. As a result, the "real" duration is much longer.
With the Chemist and Day Tripper perks, rank 2 of Implant GRX will last 8 seconds in real time with each use. With Logan's Loophole, Chemist, and Day Tripper, the duration of each use (rank 1) is 16 seconds in real time.
Keep in mind that rank 2 of this perk cannot be taken with Logan's Loophole.
The Fast Times perk does not affect Implant GRX duration.
Like all chems, the duration of Implant GRX will stack with multiple doses. This can result in a lengthy duration of slow motion by consuming many doses at once (up to 80 seconds in real time using all 10 doses with GRX rank 2, Day Tripper and Chemist).
Uses of Implant GRX cannot be hotkeyed.
The effects of the implant are not cumulative with the chem Turbo. Doses of Turbo taken at the same time will be wasted.