Modern gliders are an interesting data point regarding this. A competition glider carries water in its wing tanks to increase weight. This betters performance at high speed, which is essential for competition when conditions provide strong lift.
The pilot can release the water at will, and the tanks can be emptied in a few minutes. This happens in competition every day, either when you are arriving (to land without the extra weight) or sometimes mid-flight when conditions are worse than expected.
So, taking as an example a Ventus 3:
This glider, with pilot included, weighs around 380kg. It can carry up to 212 litres of water in its wings, for a maximum gross weight of 600kg (it also carries a few litres in the tail for cg purposes).
In summary, the pilot can change the weight of the glider from 600kg to 380 kg in some 4 minutes (meaning a 38% change in weight!). This makes the change very noticeable. When empty, the glider is more "feisty", both in that it reacts more to turbulence, and in that the commands (the ailerons, especially) are way more effective.