Max pulse rate from Smoothie is 100 000 steps/s (IIRC).

For move of 300 000 mm/min = 5000 mm/sec x 256 steps/mm = 1 280 000 steps/sec

Working forwards, with 256 step/mm and max step rate of 100 000 steps/sec = 390 mm /sec = 23 400mm/min

(I haven't even taken cosine transform of delta to cartesian, which can require up to 2.5x more steps per mm)

Remember also, minimum pulse length of step pulse from smoothie is ~3us, so even with instant drop to 0, then new rising edge at 3us, max step rate theoretically possible is 333 333 Hz, at 256 steps/mm = 1302 mm/sec. Check your external driver docs too, they usually have a minimum pulse duration for Step input - mine are 1.2us but I've see as high as 5us.

Remember also, ABB, Kuka, Fanuc etc use AC servos, not stepper motors, for exactly this reason (well also because the stepper motor itself loses torque as speed increases, so its not just about processing speed)

Just a rough look at your setup, large Linear Delta with incredibly high speeds desired - if this is for pick and place with low accuracy required (like sorting robot for food industry), try to get your gearing higher, so steps/mm is more like 25steps/mm, and drop acceleration to around 2000. Your large motors have good torque at low speed, so you want to keep motor speed low and use gearing to increase robot speed.

Ultimately, I think you need to be looking at AC servos and Mach3 or LinuxCNC to drive it.

What stepper driver are you using ? Many high-microstepping drivers have the option to do "extrapolation" where for example you are controlling them at 1/8 microstepping, but they output at 1/256. This generally won't really cost you precision/accuracy, but allows you to work around the 100khz limitation.

Edit : also your config file is a very old revision, please make sure you upgrade to the very latest config file example ( porting over your old values to the new example file ) and latest firmware, before asking for help, as the documentation requests.