The 2/21 sdk release has introduced a change where avatar validation is performed constantly while the build panel is open. For some avatar hierarchies, this needlessly increases CPU usage and reduces Unity to a near unusable state as there is no longer any cooldown period between checks.
On a related note, the Avatar Validation script could stand to be revisited for performance. The FindTypes function is static, yet returns an uncached enumerable that performs multiple type reflection calls. DotNet Reflection is notorious for performing poorly, so evaluating this enumerable every time avatar validation is invoked is likely a needless performance loss.