Hi all, I volunteer with a local nonprofit, and we recently set up some donation options, but they seem to have tanked our website speed.
Specifically, the zeffy.com form (which exists as a button and then pop-ups up when you click it) shows a main thread blocking time of 12,551 ms, and the GoFundMe widget is 6,475 ms. (There is also an hcaptcha listed for 9,133 ms. I'm not sure where that one is coming from, but I suspect an old plug-in and need to do some database optimization, perhaps.)
I've been using WP for a long time, but optimization is a relatively new concept for me.
I tried defer and rel="preconnect", and that helped a little but not that much.
Is doing an iFrame the next best option? Or is there a better way to go about this?
Any insight, links, or resources would be appreciated.

Yes use an iframe, and try to position it below the fold. If it’s a JS embed, use ‘defer’, and put the script tag at the bottom of the page.
What you need to do is to use the GIVEWP plugin. Over a hundred thousand non profits use it for a reason. It is very good , very fast and works like a charm. I am about to use it for my advocacy group to protect American technology workers from the big bad wolf.
Donorbox is pretty good last non profit I set it up for