As far as the changes, I've got those documented here.I'm from Canada and I recently just traded in my 2019 model 3 for the 2020 one.
Most importantly, am I going crazy or does the 2020 model 3 sr+ feels slower than the 2019 model 3 sr+?
Can't really say if there's something up with your car, and not sure what model you have. For us, just 2 data points for you. My 2018 LR AWD does 0-60 in 4.28 sec on the dragy, and my wife's 2020 SR+ is 5.19 sec, documented here. Both exceed what the Tesla.com pages said when we placed each of the orders, car details in my signature below.