There are fast science victory guides in the
war academy tab, it's a bit complex to learn just from friendly comments as there are so many factors and things.
A few tips:
1. Learn worker stealing and do it fast
2. Play on Deity for the best times for the easy acquisition of the AI gold
3. standard, 4-city, fast national college, max growth is the main idea. Use most of your trade routes for growth and make sure you acquire the happiness to keep it going for as long as possible.
4. Save, steal, or borrow money to buy most of the science buildings as soon as you get the techs
5. Always open rationalism as soon as possible and get the +2 science from specialists, and 17% to universities as quickly as possible.
6. Pick freedom ideology and get down to buy space procurements
7. Next fill out rationalism, then aim for the 25% off gold purchases tenet in commerce
8. Save your scientists, buy as many as you can with saved faith, then run research for 10 turns at the end and burn them all one turn at a time to research all the critical techs for spaceship very quickly, then get gold/aluminum from the AI and buy most of them to launch quickly.
9. I think most players that get the fastest times are playing either Korea or Babylon and plant one early academy if Korea.
This is the general idea, but even doing all that it will take practice and luck to get "the fastest".
Once you have the main steps down the biggest thing that makes the difference between T240 SV and T190 is luck: AI behavior, how good your start is, warlike AI and whether or not they can be bribed off, how long you can get away with a puny military, etc. Most players play standard/pangaea for the best odds as you meet everyone quickly for early trades and won't be stuck alone with a warlike neighbor.
If you want a competitive time I think most players reroll the map till they get something like a coastal/river/mountain start with solid resources. You absolutely have to have coastal for the fastest growth as ship food routes are far superior and the mountain observatory bonus is huge in a capital you are already feeding, planting academies, getting NC, etc. River is the lesser requirement but having riverside boosted food tiles and later the hydro plant and the extra gold from running from a river city is all very nice.