When I download with IDM I cannot browse in the Internet or use other internet applications. Can I limit IDM speed somehow?

In default mode IDM tries to use all avaliable bandwidth. This why other internet applications may work slow.

IDM has Speed Limiter feature. You can access it using "Downloads→Speed Limiter" IDM main menu item or "Speed Limiter" IDM tray icon right click popup menu. This option allows you limiting speed of each download. Use "Downloads→Speed Limiter→Settings" IDM main menu item (arrow 1 on the image) to set speed and select other options and then use "Downloads→Speed Limiter→Turn on" IDM main menu item (arrow 2 on the image) to start Speed Limiter.

Open IDM speed limiter settings

Set maximum download speed in speed limiter

Please note that you can limit only speed of individual download and not IDM total download speed. If you start several downloads each of them will be limited to a speed you set, but total IDM download speed will be number_of_downloads x set_speed.