'tribulation' (thlipseōs) is a word used in a number (45) of texts in the NT meaning 'hardship' and often used to mean 'persecution' of Christians. (eg. Acts 11:19, 2Cor1:8) Revelation was written at a time of persecution and expectation was for this to increase and culminate in all Christians being killed and going to heaven.
As an aside, the "the" as in "the great persecution".is just grammatically called for as this sentence follows a colorful description of that persecution. Much like the 'the' in Rev 1:9 is grammatically called for.
"I, John, your brother and partner in 'the tribulation' and the kingdom"
Yes the writer anticipated this persecution to come to a final head, so in that way the persecution anticipated in Rev 7 was greater than previous persecution.
Well, It didn't happen, in fact in time the Christians became the persecutors. And here we are 2000 years later with some here still trying to shoehorn it into JW chiliast doctrine.