1. Your X motor appears to be mounted incorrectly. If it is not flush with the lawsy carriage, it will wobble, resulting in backlash on your X axis.
2. Your error is not consistent around the circle, but if we draw a line through the circle, we almost get perfect symmetry between the sloppy areas and the areas of perfection.

Check your belts and ensure everything is rigid. Any movement beyond what the motors call for is a bad thing. Most likely, your X motor is loose, and your hotend has some slop as well.

Speed isn't really a problem. I can print something like this at 100mm/s with my fan turned on (or enclosure off). A lot of the issues you are experiencing went away when I switched my SD4 to a direct drive system where the Y motor moves the shaft via a coupler.

I have found that the ideal temp can vary from one roll to another even with the same type of plastic. doing a temp calibration is now a regular part of my calibration routine when changing rolls. I keep a record of what works best for each roll I have - feed steps/mm, extrusion multiplier, & temp calibration. (I have 6 rolls of PLA in various colors, and 4 rolls of ABS in various colors - each one has different settings)

and yeah, as carl said - speed matters, too.

Lately when I print circles, the filament sometimes does not stick to the last layer (see picture).  How do I fix this?

Slow down. Think of a string of hot plastic like a rubber band. If you stretch around a bend real fast then it will pull away from the bend as it cools. If you slow down then it will cool more as it is placed which will allow it to stick better. Depending on your software if it allows do inside perimeters at 25 or so.