It's a burden on the DNS resolver side. 4096- or 8196-bit key operations aren't exactly cheap - and the time taken for the public key operation grows superlinea

It's a burden on the DNS resolver side. 4096- or 8196-bit key operations aren't exactly cheap - and the time taken for the public key operation grows superlinearly with the bit count (depending on the specific variant of the modpow algorithms in use, somewhere between O(b^1.585) to O(b^2)).