If you are using Windows, I would suggest (in line with Petr's answer) lowering Blender's priority.

Rather than limiting Blender to 50% and leaving 50% available for other programs, this lets Blender use all of the CPU not being used by other programs (meaning it will run near 100% if it's the only program running).

Press Ctrl+Alt+Del to open the Task Manager, click to the Processes tab, right-click on Blender, highlight Set Priority, select Below Normal, then accept the dialogue by clicking Change Priority.

NB: When you close and re-open Blender it will automatically be set back to Normal priority so you have to change this every time you want to run it in the background.

Avoid making more drastic priority changes without understanding the effects.

If you are using Windows, I would suggest (in line with Petr's answer) lowering Blender's priority.

Rather than limiting Blender to 50% and leaving 50% available for other programs, this lets Blender use all of the CPU not being used by other programs (meaning it will run near 100% if it's the only program running).

Press Ctrl+Alt+Del to open the Task Manager, click to the Processes tab, right-click on Blender, highlight Set Priority, select Below Normal, then accept the dialogue by clicking Change Priority.

NB: When you close and re-open Blender it will automatically be set back to Normal priority so you have to change this every time you want to run it in the background.

Avoid making more drastic priority changes without understanding the effects.

If you are using Windows, I would suggest (in line with Petr's answer) lowering Blender's priority. Set to a low priorty, Blender will only use as much CPU as is available. If it's the only program running, it will utilize 100%. If someone is playing a video game or something else intensive, the percentage will drop down and you won't notice a negative effect on the performance of the other programs.

This is simply the best way to render in the background - limiting to 50% unnecessarily limits the power when the computer is idle.

Press Ctrl+Alt+Del to open the Task Manager, click to the Processes tab, right-click on Blender, highlight Set Priority, select Below Normal, then accept the dialogue by clicking Change Priority.

NB: When you close and re-open Blender it will automatically be set back to Normal priority so you have to change this every time you want to run it in the background.

If you are using Windows, I would suggest (in line with Petr's answer) lowering Blender's priority.

Rather than limiting Blender to 50% and leaving 50% available for other programs, this lets Blender use all of the CPU not being used by other programs (meaning it will run near 100% if it's the only program running).

Press Ctrl+Alt+Del to open the Task Manager, click to the Processes tab, right-click on Blender, highlight Set Priority, select Below Normal, then accept the dialogue by clicking Change Priority.

NB: When you close and re-open Blender it will automatically be set back to Normal priority so you have to change this every time you want to run it in the background.

Avoid making more drastic priority changes without understanding the effects.