Can I adjust print speed on the fly?
yes
Print speed is a setting that can be altered by just turning the click-wheel of the Ender 3. You don't need to push it to gain access to menus. A turn to the right does increase the speed, left lowers it. It is applied only some moments after
stopping the turning - then the firmware does inject a M220 command as the next line. This means the current running movement is ended with the last set speed, the new speed is set and the following command will be done with the new speed.
As towe correctly states, one can also send a M220 command to the printer via a terminal, but then you need to have one set up before the print starts, as plugging in a terminal will reset your printer and abort the print!