Security is the top priority. But being useful is also important; if nobody can use it, nobody benefits from that security. Acceptable performance is very important for being useful, especially when tunneling layer 3 packets, where latency has rippling effects. The prior non-kernel WireGuard for Windows simply was not sufficiently useful for real world workloads people wanted to run, because of the lower performance, both on servers and on laptop wifi alike.

A big part of the WireGuard project since the beginning has been trying to figure out how to do high security tunneling that also performs acceptably. It's easy to do one without the other, but doing them both together has meant thinking about a lot of fundamentals, from the protocol state machines on up. It's hard to do tunneling in the kernel at high speed while still maintaining a strong security posture. That's a principal challenge the project endeavors to solve.

More generally, it's worth noting that cryptographers also care about performance quite a bit in things like symmetric crypto. We know well how to make a good cipher now, but making one that also performs at increasingly high speeds remains an open area of research, with whole conferences, such as FSE, devoted to it.