According to a report from The Wrestling Observer Newsletter, the reason why the John Cena-Undertaker match at WrestleMania 34 was so quick was because WWE officials were informed that it wouldn't be possible for Undertaker to wrestle a very long match this year. They were apparently informed of this several weeks before WrestleMania.
This is also why the decision was made to not really hype the match up as much as fans might have wanted. WWE officials didn't want to put a lot of hype into it, only to deliver a quick match. So the decision was made to make the fans think that Undertaker would probably at least show up, albeit with some doubt, but they didn't want anybody to expect a stellar in-ring performance.
As for Undertaker's upcoming Casket Match against Chris Jericho at the upcoming Greatest Royal Rumble event, it's very likely that this will be a rather short performance as well.