Also DNSSEC doesn't have to use RSA: https://blog.cloudflare.com/a-deep-dive-into-dns-packet-size...
They are last-generation legacy elliptic curve operations. Cryptographers have moved on to significantly better, safer designs.
It's 2016 and the vanguard of cryptographic modernity in DNSSEC is P-curve ECDSA --- if you use it, you're taking a risk that some deployed resolvers won't understand your zones. The breaking-change new crypto in DNSSEC is already obsolete.
Plus, as far as big number arithmetic goes, it's relatively easy to implement.
I believe someone here said you said one shouldn't rely on DNSSEC anyway?