This is ONLY in certain cases not for every fighter match up.
All transitions are 100% blockable even if you have slow reaction speed UNLESS when any of the following set of variables are met
-momentum based transitions
-large stamina discrepancy or GA
-large stat discrepancy
This is one of the only issues I have with the grappling in UFC2. If you have a someone with mediocre clinch grappling vs GSP it doesn't matter how fast you are. You literally have to pre deny every transition in the clinch. Same goes for the ground game. Having a khabib on top of you while playing anyone not named Nate Diaz and you will need to pre deny certain transitions.
Off the top of my head the pre denial transitions on bottom would be like stacked guard, crucifix, posture up half guard.
However there is a positive to this as well that adds to depth of the ground game. For instance, when you are on bottom in half guard, the transition to guard is pretty quick. MOST people can't reactively deny that transition from the top ESPECIALLY if you have a guy with high off the back stat. This is where the mind game comes from - if the guy on top wants to finish the fight he will care that you keep slipping back into guard everytime he passes to half guard so he will want to deny it. As soon as you think he's made the decison to 100% deny the guard transition* you do a back take in HG. (This doesn't work versus players with exceptional reaction speed OR who just care about top control and don't mind being back in guard)
There are 3 ways to deny a transition
1) Reaction - you see it -> brain interprets which way you need to flick -> you deny. To be able to do this confidently you need to know what your capable of. You need to know at what stamina/GA/stat difference can you react to confidently.
2) *100% Reaction - You need to be fast. As SOON as your opponent twitches you are flicking a certain direction. This is the most important denial and most interesting. It causes a lot of the mind games on the ground. You are predicting where they will go based on prior transitions or prior fights or just a hunch. An example for this is in mount - If I mount someone and I have a decent stamina disadvantage I know that I will not be able to reactively deny their half guard escape (unless stat discrepancy is in my favor). So I pick a side, lets say left and as SOON as he moves I flick left. Doing this denial shaves off the time it takes your brain to process which way to go. This type of denial is where most reversals come drom. Also to note in laggy matches this is the primary type of denial.
3) Pre denials. Like I said earlier, you need these for momentum denials and last resort scenarios. They do add to the overall psychology of the ground game. No one on planet earth (WHEN NO VARIABLES ARE MET) can reactively deny the mometum or 100% GA/Stamina difference transitions. You need to flick these before any visual prompts come up. Every mometum based transition is deniable you just need to practice the timing.
I really hate when people complain about the ground. It's god damn beautiful. I think one issue is that the ground is complicated and has so many layers of knowledge that people haven't discovered yet. Another issue is the players who take 0 risk and lay on you looking to win on points. The Ben Askren method. I'm pushing hard at getting quicker ref stand ups for the next game so theres more sense of urgency.