I’ve always used the Accelerated setting (In RepG) ie. FeedRate:80 and Travel:150 without problems in general. I do however OFTEN use the Speed settings on the LCD screen during print (UP and DOWN buttons) as necessary depending on the specific object and stage of object print (some layers of an object may get better results slower than the rest) and material type. I gather that the numbers are a percentage of your Feed and Travel Rates that you set when you generated the gcode initially, so pressing DOWN to example 0.50 would halve your Feed and Travel Rates whilst pressing UP to 2.00 (wouldn’t recommend that high as belts would more than likely skip!) would double your rates. Comes in quite handy!
Need Firmware 7.5 for those functions though. 7.2 doesn’t have it.