blocktimes are every ten minutes. Theres no possible way it would have any noticeable effect. I doubt you could delay them for more than a second or two
Discussion
You are making lots of assumptions
Like that spammers would pay more money to spam in OP_RETURN then the SW for the so called health of the network.
It’s not that knots would prevent all spam but it definitely would slow it down and make it tougher for spammers to propagate throughout the network.
Look these guys already have libre relay and disregard OP_RETURN limits but their transactions don’t get picked up by the network because no one runs Peter Tards spam blaster.