Not sure I'm being throttled

I had Cox internet for over 20+ years at its lowest speed of 300Mbps/30Mbps.  It was quite fast even at theose speeds. I recently switched to Spectrum knowing I would have to eventually do so.  I got the Internet 500 plan which upload speed is 20Mbps, slower than Cox. I was expecting it to be slower than Cox but not that noticeable.  It's not all day but at certain times. I'm the only one using the internet.

Could it be my Router/Modem which is an Arris or am I being throttled?  I hope that's the correct word.