If you travel often, it makes sense to invest in fast track. You skip all the security theatre. Also, for example TGV requires you to be at the check in 20 minutes before. You’re a bit late, you’re screwed. Even if the train hasn’t arrived yet.
Where?
For TGV Inoui (the main brand) you have to be in the train 2 minutes before (written on any ticket, also here: https://www.sncf.com/fr/offres-voyageurs/voyager-en-train/tg...), and that's all. People flow through the gate up to the last minute. It has been like this for at least a decade.
For TGV Ouigo (low-cost brand), it seems to be 30 minutes according to https://www.ouigo.com/faq?question=ai-je-pense-a-tous-avant-...
I never understood this constraint with Ouigo though; it's obviously possible to skip it as shown by Inoui trains... and both are operated by the same company from the same train stations.
However in practice most people respect the 30 minutes, so that you can arrive 10min before the train and get your ticket checked without waiting in line. This is not guaranteed. But often works. I take a lot of OUIGO and never respect the 30 min, because you often wait 10min in line. Once I arrived 5min before departure, had my ticket checked but had to run in front of the correct car. When train arrives nobody is allowed to pass the checkpoint.
For the InOui you can always come 5min before departure, because tickets are checked on-board so there is no wait line.
In Brussels.