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This clock-shaped icon is at the top of the IDM window. It's near the top of the Scheduler window. Click the {"smallUrl":"https:\/\/www.wikihow.com\/images\/8\/82\/Android7dropdown.png","bigUrl":"\/images\/thumb\/8\/82\/Android7dropdown.png\/30px-Android7dropdown.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an Android icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} arrow to the left of the "Download" text until the number next to "Download" is 1. This will increase the speed of one specific download drastically. You may want to change this setting after downloading a high-priority file. It's at the bottom of the window. Doing so saves your settings.

Summary:
Click Scheduler. Click the Files in the queue tab. Decrease the number of files that can download at the same time. Click Apply.