I'm looking for an old script where you switched from knife to another weapon (preferably a nade) really really fast by holding down a key (usually q)

Pretty sure "lastinv" was used and maybe "impulse" command. The script did not have any advantage in-game, it only switched from one weapon to another really really fast by just holding down a key.

Get in your console and use the commands:
_cl_fastswitch 1
hud_fastswitch 1

This should switch from your knife to primary weapon, relatively fast.

You could use some binds in your console like:
bind wheelup slot1

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You might also be interested in some binds for fast buying as well:
http://gamebanana.com/scripts/6446