Is there a minimal approach speed for airliners during rush hour?
ATC can not only ask for a minimum speed, but rather instruct each aircraft to (more or less) exactly fly at a particular speed. This is essential for maintaining separation. As J. Hougaard pointed out in the comments, the speed on the last 4 NM is always up to the pilot to slow down to final approach speed.

It is the duty of the pilot to evaluate if it is safe to comply with this restriction. In case the aircraft cannot fly as slow as asked for, the pilot can simply reply UNABLE and ATC will either try to make it work or instruct the aircraft to go around.

Some airports also publish speed restrictions on the approach charts. This is an example of a minimum speed from the ILS approach into 01L/R at Stockholm-Arlanda (ESSA):



More precise speed restrictions are for example given on the STAR charts of the (now closed) Istanbul-Atatürk airport (LTBA):