I assume you refer to the jettison of the garbage pallet containing the old batteries, which came in at about 2.9 tons. Those were jettisoned using the Canadarm2.
The Canadarm2 has the following operational limits:
Speed of Operations
- Unloaded: 37 centimeters / second (1.21 feet / second)
- Station Assembly - 2 centimeters / second (.79 inches / second)
- EVA Support - 15 centimeters / second (5.9 inches / second)
- Orbiter - 1.2 centimeters / second (.47 inches / second)
(Numbers from Canadarm2 and the Mobile Servicing Subsystem)
I am not exactly sure where on this range the pallet falls - but presumably somewhere between 2cm/s and 15cm/s. The upper limits is 37cm/s.
Orbital velocity of the ISS is ~7660m/s. That means the pallet can only get about 0.005% of change of velocity from the arm.
Combine this with the fact that the pallet is very dense and has a small-ish cross-sectional area, the ballistic coefficient is rather high, so it doesn't experience much drag (compared to a large, light object).
Going by this Q&A, jettisoning stuff by hand yields velocities of about 0.6m/s, but only for stuff that is significantly lighter than 2.9 tons.
@Uwe already gave a good comparison with the Shuttle, which needed 90m/s for the re-entry burn. The jettison was done with 0.37m/s.
You can not simply "push harder" because of the operational limits of the Canadarm2 and the human body, depending on who does the jettison.
And with such a small change in velocity, it takes a while to de-orbit. Which doesn't really matter. Making it de-orbit faster would incurs high costs for no apparent benefit.