Internet speed is very slow

For the last few days, my internet speed become very slow between 6pm to 12am. During day time the speed is normal. I'd try turn off the router and turn it back on, it didn't help. Speed test shows my download speed is at 1.48mbps and upload speed is at 0.94mbps. Ping is at 12. It's on wifi. It's so slow that you can't even watch Netflix and YouTube.