This would ruin capture ships. I would reccomend having a large escort force with the destroyers as it is the best counter.

True enough, although half the time this means focusing all firepower on them, and with the speed they capture at, it's usually over before i can muster the counterattack, let alone even realize what's happening. Simply put, they don't die fast enough.

Aside from which, i'm not advocating completely negating capture ships. all such an "upgrade" would do is bring their capture speed back down to their original values (assuming that the opponent has all 3 capture speed upgrades researched, but idk if the AI does that or not)

It also does not help that there is no "mass attack capture ships" order, unlike repair ships which do get one. which would likely be an alternate solution.

Of course my experience is based on what are deemed "normal" game settings, where the AI apparently gets some form of assistance (which i did not realize until i dug into the match settings), which means that by the time i get a destroyer, the AI (typically) has an army of frigates, including several capture ships (this is playing against the Vaygr/Hiigaran, the other two factions are their own nightmare), so what was supposed to be an equalizer is quickly turned into an ironic escalation of the original problem.

I fully agree with that.