As others have said, it very much depends on the airport itself, indeed rather more so than the overall security regime of any given country... Some US airports have the physical space or layout to make transiting pretty painless (eg, both Dallas and Houston here in Texas usually have dedicated transit security passes right next to the transit bag drop, and you're often through both in the twinkling of an eye; and I've always found, eg, Atlanta, Cincinnati, and even Washington similarly easy). Older airports with layout problems often dump you full landside after bag drop, and then you have to crawl through the general security re-check (eg, JFK, Newark, Miami). Others are a mix, depending on terminal / airline mix (Chicago can be absolute hell, as can Boston). The overall pain is that it can be difficult to get get precise advance information as to exactly what to expect for which flights, which airlines, which terminals....... I've actually missed not-very-tight connections at Newark!
My experience in recent years of traveling several times a year between USA and Europe - from and to various destinations in both - in that the USA has improved overall in both efficiency and courtesy, whereas chunks of Europe have gone the other way: LHR and LGW are, in my opinion, total nightmares of slovenly overcrowding, manned by Little Hitlers (And I have both US and Brit passports..). CDG is a hell-hole too. I prefer, if I can make the routing work, Madrid or Amsterdam (though both involve a hell of a lot of walking....) or my personal favorite, little Geneva.
AGB