One of the biggest problems in high-octane, page turning thrillers is pacing.
Often the action unfolds too slowly, or too quickly.
Counter-intuitively, the climactic action scenes take longer on the page than less climactic scenes. We talked about this in the context of one of my favorite movies, Heat, some time back.
Recently I've noticed a tendency for writers to interrupt early action scenes with explanations. How the main character got to this place; how s/he knows that snake venom will slay zombies; how s/he learned what is needed to fend off a shark attack.
None of the info about how you learned to fend off sharks is needed in a scene where you're actually fending off sharks. Action means just that: ACTION. Save all the explanations for later.
In fact, after Our Hero/ine slays the zombie and fends off the shark, you'll need a break in the pace, and that's a great time for one of the other characters to say "hey, how did you know that about snake venom?" and get the info in that way.
Exposition and backstory have a place in thrillers; I'm not saying they don't. But they need to bracket the action, the high octane scenes, or they'll kill the pacing.
I would estimate that of the fifty or so manuscripts I've requested in the last twelve months, at least half have had pacing issues. I recall agent Jenny Bent tweeting something like that stat as well.
So, how do you fix this? One of the tricks of the trade I hear from my clients is that writing the book as a screenplay helped with pacing a lot.
Watch how writers you admire get the pacing right, and then do as they do.
I'm sure the comment column will have some good suggestions as well.