IPSec is somewhat similar to how wireguard work actually, it relies on IPs and static encryption keys. Not too hard to configured, see for example the manual keying documentation of slackware: https://book.huihoo.com/slackware-linux-basics/html/ipsec.ht...
ISAKMP/IKE is then used on top to manage the IPsec keys and parameters. This is where a lot of the complexity comes in, tons of parameters, modes, etc. etc.
So if all you want is to secure communication between two IPs and can securely exchange key material out of bands, manually keyed IPsec is not very complicated.
Also, even the IPSEC config without IKE is way more complicated than a Wireguard config, with seriously sharp edges. Just look at that config you linked to. No one should ever need to know what AH and ESP are, but if you don't you very easily can configure IPSEC in an insecure manner.
No, it's very fast because the ChaCha/Salsa20 stream cipher uses common CPU instructions and runs fast in purely software, whereas AES requires things like S-Box computations which is slow in software but fast when implemented as accelerated instructions in hardware. There are IPSEC software stacks using AES acceleration that runs just as fast, not to mention IPSEC hardware offload.
OpenVPN is slow due to architectural constraints, but IPSEC doesn't suffer from that at all. IPSEC tunnels with PSK is also absurdly easy to configure, either on Linux, or a router, what it doesn't offer is native NAT traversal.