Facts speak louder than words. I don't say "Definitely", but Kongo is probably the fastest culture victory civilization in the current version(PvE), not in late stage, but very early stage.
Forest and rainforest start bias is very powerful because of Magnus, this represents the early wonders can be easily taken by the players. Then +50% GPP of writers, plus four extra slots. As long as your technology arrives Printing, it's all over with Level 4 Reyna. That is called "pure book victory", the fastest record in Chinese forum is T86(peaceful culture victory)
https://tieba.baidu.com/p/5587664945, and it is likely that even the warmonger based culture victory won't be that fast.
In addition, Kongo is likely to be one of the strongest civs in OCC science victory.
I'm pretty sure that is not on deity or on any significant difficulty level (great library, etc.). ~260 tpt is hardly enough to take down deity 8 player maps (esp. without routes most likely as the map is still unexplored) without conquest. If we are talking about settler level then the conversation is pointless.
With the right conditions the turn times can go lower I suppose, by combining the "pure books" with relics + reliquaries + St. Basils... and on lower difficulty AI will not reach enlightenment. But really, on lower difficulties if you conquer I'm sure you can go lower than 86.
Lily's right Kongo is weak on deity. Ridiculously so early game... UU does nothing, as they lose to mere swordsmen (let alone horses and knights) due to the difficulty bonus and take huge damage attacking cities despite taking reduced damage for bombardments. X-bows will still chew them up. Oh, but they probably have the fastest peaceful cultural victory possible as long as they get lucky with reliquaries (which is a factor outside your control). I've tested and too bad that Basil's stacks additively, not multiplicatively with reliquaries... so it will be 300% + 100% = 400% tourism for relics... which is significantly less than the 600% I was hoping for initially. Nevertheless palace (5) + Michael (2) + Basil (3) and if later game + natural history (4) and luckily Apadana (2) and free bank from GM (2), Kongo has the highest possible number of relic slots (18) in a single city, makes for a possible OOC cultural victory even on deity (although I am more concerned whether you will have enough space to build 17 Mbanzas + theatre square, for the apostles as a lot of times the game will not let you plant forest due to strategic resource, and luxes/districts/wonders may end up taking too much space)... getting the GM in question will also be difficult OCC (if you already built a bank, will that GM still build you the 2-slot bank?)
All in all they are a tier-C+ to B- civ if you go by turn times. There are worse civs to be had (looking at you, Georgia) by far. For top-level play, they will be quite bad due to being unable to get holy sites and so miss the double science from dark age.
That said, they are so much fun to play. Free apostles means you can abuse heathen conversion quite often... usually you can get 2-3 with that promotion (and that in the right games, it makes building a military by yourself unnecessary, as well as getting eurekas and historic moments). Building huge cities and flipping AI cities due to golden ages from those wonders you built using your superior city size (and larger domains with more stuff to chop and harvest) is really enjoyable that when I go back to play any other civ it feels like I'm annoyed it takes so much longer to build stuff and my cities seem so small. City flipping with a vanilla civ happens so rarely when compared to Kongo... I just ENJOY chain city-flipping... and once the city flips you can just route in food and build mbanzas, and presto! Size 10+ in a handful of turns and ready to flip its neighbors. You can settle aggressive cities for that purpose, and fresh water is not an issue as Mbanzas takes care of all housing problems and gives +2 food +4 gold to city center without being worked. On non-deity difficulties where AI is slow to get multiple governors I suppose you can completely conquer an entire civ peacefully.
On deity, I feel somewhat not so bad if AI forward settles me as I know that one my capitol grow to 20+ the city will flip eventually. This makes early expansion not so crucial and I can focus either on getting a friendship (by builder first improving lux for gift) or defense (archers). I've had a game where I did not build a single settler until ancestral hall was finished. (With magnus and a couple of workers though, you catch up really quickly thanks to 100% production bonus + 100% chop) and still won that game easily without any conquest.