Q. Why does it take so much time to enable or disable DNSSEC?
DNS has a cache, and cache is controlled by the TTL. It takes long because there is a need to go on to the next process after the cache used before enabling or disabling DNSSEC has disappeared completely.
Wait until the process is completed.
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Because TTL of DS records which are registered with the higher zone is also affected, the time to enabling will not be shortened even if the size of TTL of your zone is reduced.
Note that handling of higher zones is outside the capability of IIJ.