I'm sure that it's well known that FreeBSD iwm lacks 802.11n and 802.11ac support.
I believe OpenBSD supports 802.11n on iwm (I couldn't try OpenBSD as it doesn't boot on my laptop). A commit is here: https://github.com/openbsd/src/commit/d1ea7b2160ed2eb0df49e1e2db31cabcceb666cb
There is an abandoned GSoC project for FBSD 802.11n/ac: https://wiki.freebsd.org/SummerOfCode2017/Porting80211nAnd80211acToFreeBSDiwm
Would someone be willing to write a FreeBSD patch for at least 802.11n on iwm (if not ac)?
I doubt ac support will come soon, but it doesn't look too hard to port the OpenBSD changes for initial 802.11n support.