Reduce printing speed  

Change speeds or MVS

If you want to reduce all speeds 10% proportionally, you have to edit the individual speeds (Print settings->Speed). If you'd just like to cap the MAXIMUM speed, you can do so using Maximum volumetric rate. That will set an upper limit on how much filament is fed into the hotend. If your individual speeds are low enough to stay below this level, they will be used. If they result in a higher flow, the MVS limit will be applied. Think of it as a top speed governor. (Set under Print Settings->Speed->Auto Speed->Max volumetric speed.)

Remember that the slicer and printer settings are separate. If you limit speeds in the slicer, the gcode will be created using those speeds. If you then adjust while printing, that is a multiplier applied to whatever is in the gcode. Also remember that changes on your printer are stick and will remain set until you reset or power cycle the printer if you don't reset them in gcode.

Thanks!

I will stick by reducing the Print Settings Speed this volumetric speed sounds more complicated 🤪 . Thank You!

M220 Set Speed Factor

Another (advanced) way is to add a gcode in the startup gcode to set/reduce print speed in PrusaSlicer gcode startup:

M220: Set speed factor override percentage
Example:
M220 S80 ; Will reduce all speeds in gcode to 80%

See reprap.org definition.

Be adware that this setting is in effect until changed or power off/on as @bobstro mentions.

Nice

This is what I was looking for. I will give it a try, Thanks