I could not use your link. G-code can directly attached to a post using the upload icon. As for feedrate, the limit for Z movement is slower than the X and Y movements. The first change to make is to execute the following g-code once:
M203 Z8 ; Limit feedrate to 8mm/s (480mm/min) M500 ; Save the changes to EEPROM
Next, if you are using the guffy post processor, you may want to update to Don Gable’s version. He has done substantial work including some settings that deal with feedrate. See this topic.
How do I raise the current on my z-motor if I want to go down that path?
This will depend on your stepper driver. For many boards you can make the change either directly in the firmware or by using g-codes. See the g-codes M906 for TMC drivers and M907 for other drivers. You likely have to send an M500 to save the new setting. Some drivers like the DRV8825 and the A4988 require you to physically adjust the current using a voltmeter and turning a potentiometer on the board.