Assuming an ideal capacitor, a real world solar panel will give
slightly more current when short circuited, so the rate at which
charge is accumulating is initially high. But this does not mean that
the rate that energy is being stored (i.e., the power) is high. It
takes more power to push a fixed amount of charge against a high
voltage than a low voltage.
If you play with the equations V = C [integral of] i dt, and E = 1/2 C
V^2, you'll see that for constant current, the energy in the cap is
proportional to t^2. So if you store 1 unit of energy between 0 and 1
second, you'll store 10^2 - 9^2 = 19 units of energy between the 9th
and 10th seconds.
The power flowing into the capacitor will continue to increase until
physical constraints (the panel is not an ideal current source) take