<p>I am running an Ender 3 pro with an Octoprint connected. I accidentally set the print speed too slow in Cura and the print will take very long. Is there a Marlin command I can issue to the printer to speed it up without stopping the print?</p>

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!