DIAGNOSIS A drastic slowdown is almost always a clogged filter. The activated carbon inside gets saturated with the gunk it's trapped, and the mesh screen on the bottom can get blocked with fine sediment from your water.
FIX First, check if it's time for a filter change. The standard lifespan is about two months or 40 gallons. If you're past that, just replace it. If it's a relatively new filter, it might be clogged with particularly heavy sediment. Try this: remove the filter from the lid. Hold it under a strong stream of cold tap water for a full minute, rotating it to rinse all sides. This can flush out some of the surface-level sediment. Tap it gently on the counter to dislodge particles.
IF THAT DIDN'T WORK Consider your water source. If you have very hard water or a lot of iron, it can coat and clog the filter much faster than average. You might need to change filters more frequently. Also, always start with cold tap water. Hot water can cause minerals to precipitate out faster and clog the pores.
HEADS UP Never try to speed up the process by poking holes in the filter or scrubbing it aggressively. You'll damage the delicate mesh and release the carbon granules inside, which will end up as black specks in your water and ruin the filtration.