I also got this problem of the first runs being extremely slow.
I had no speed issue playing wiht the manual command in repetier so I guessed the gcode was giving bad feedrates.
It happened that if I switched the feedrate unit in the Marlin profile of ESTLCam from mm/s as you suggested to mm/min, it would print at the expected speed.
Then I checked in the Marlin code and docs to find out what was the ‘standard unit’ for feedrates, and found nothing but mm/min.
Maybe I missed something but I wonder how you got ESTLCam set in mm/s output valid feedrates for Marlin ?