Those cheap Chinese outdoor solar lights are (usually) made with inferior solar panels and batteries. Which is why they're cheap
Keep in mind that the efficiency of solar panels decreases with time; some panels lose as much as 5% of efficiency per year. Considering that the batteries in these systems are bottom of the line, as the marginal efficiency of the panel drops and the battery deteriorates from cycling, they're likely going to go downhill in just a year or two.
Also, the efficiency of the system is partly determined by your location (higher latitudes get less sun) and whether the panels are kept clean. Even a modicum of dust, dirt, or water deposits can make a significant difference in how much power is generated.
I tried a set of those a year ago as I was testing things for my railway, and they barely "glowed" when they were new (not like actual lights, more of a glow-worm kind of light) I decided to go hard wired--I can get bulk WW LEDs with resistors from eBay for just pennies a piece. I'm powering several dozen these with a laptop power supply, get bright light and have had no problems thus far.