As @fred_dot_u mentions, Simplify3D has the capability to do this, but you can achieve the same effect by slicing the file twice, once at 40 mm/s and once at 50 mm/s, and then manually combining the generated G-code using a text editor. You should be able to find the point where it transitions from printing the cube to printing the circular structure by looking at the Z-height, and you can simply copy-paste the G-code from that point onward from the 40 mm/s file into the 50 mm/s file.
For instance, you might look for a point like this in the G-code (Cura):
G1 F3600 X113.543 Y94.098 E105.96969
G0 F7800 X113.743 Y91.369 Z2.700
or this (Simplify3D)
G1 X175.981 Y74.019 F2880
; layer 25, Z = 1.869
; inner perimeter
G1 X162.982 Y57.870 F7500
G1 Z2.069 F900
Note that the both include a comment line (a line that starts with
;) to indicate the layer. Everything before that you should copy from one file, everything afterwards from the other.