As your question suggests, the original Greek word “quickly” means “shortly, that is, without delay, soon, or (by surprise) suddenly,…” (from Strong’s Concordance). 

Now, let's look at 2 Peter 3:3-9 (Young’s Literal Translation), “This first knowing, that there shall come in the latter end of the days, scoffers [not sincere questioners, doubters], according to their own desires going on, and saying, ‘where is the promise of his presence? for since the fathers did fall asleep, all things so remain from the beginning of the creation’, for this is unobserved by them willingly, that the heavens were of old, and the earth out of water and through water standing together by the word of God, through which the then world, by water having been deluged, was destroyed [the flood in Noah’s day] and the present heavens and the earth, by the same word are treasured [reserved – stored up], for fire [symbolizing destruction of the present evil systems, Zephaniah 3:8-11], being kept to a day of judgment and destruction of the impious men. And this one thing let not be unobserved by you, beloved, that one day with the Lord is as a thousand years, and a thousand years as one day; the Lord is not slow in regard to the promise, as certain count slowness, but is longsuffering to us…”      

We see that what seems like a long time to us, two thousand years, is but a couple of days with the Lord. God is an exact timekeeper. Just as there was a set time for Jesus’s first advent [Galatians 4:4 (NIV), “when the set time had fully come, God sent his Son…], there was also a set time for his return

So in Revelation 22:20, Jesus comes “quickly”, or in other words, without delay. It was exactly at the pre-appointed time.  Jesus had a specific work at His first advent –  “who gave himself a ransom for all, to be testified in due time.”, 1 Timothy 2:6.  So at his second advent, he has the work of bringing in the long-promised kingdom when that ransom for all will be testified to all. All will then have the opportunity to believe and be blessed by it. “Thy kingdom come, Thy will be done on earth, as it is in heaven.” Matthew 6:10