I fly an elderly jet and making fixes at a particular time isnt sonething that we can do easily. ATC know this and "Slow to 210 for spacing" or "maintain 250" is as good as they can make it for us. Its ATC's job to manage traffic flow and they should know which aircraft can do what.
In saying that an A320 should be perfectly capable of time at fix. If crews of a particular airline are showing poor airmanship then a quick phone call to the CP should fix it.