DIAGNOSIS This is almost always a sign of battery degradation. Lithium-ion batteries lose their ability to deliver peak current over time and with charge cycles, which directly translates to lower motor RPM.
FIX There's no fix for a worn-out battery in this sealed unit. You can try to maximize remaining performance: always charge it fully before use, use it with room-temperature ingredients and plenty of liquid to reduce load, and pulse in very short bursts to avoid overtaxing the weak battery.
IF THAT DIDN'T WORK If the performance drop was sudden, check for physical binding. Make sure no fibrous material (like spinach stem) is wrapped around the blade shaft, impeding rotation. Disassemble and inspect.
HEADS UP These portable blenders have non-replaceable batteries. The typical lifespan is 1-2 years with regular use. Slow, weak blending is the primary symptom of end-of-life. It's not broken, per se, but its useful life as a portable device is over.