Speed issue with Slic3r and Marlin
No, you did not set all the settings to the same speed, there are 2 different print speeds defined:


Infill Solid: 60 mm/s
First Layer Speed: 30%


This means, that some walls will be printed 10 mm/s faster than the others, and that first layer should be printed at 30% of 50 mm/s, so at 15 mm/s - try to define it as a speed. Make sure to save and apply the settings and only then slice your model, as you might have sliced the model with the previous settings.