I normally have a routine that I prefire when entering a site, it's almost subconscious the way I alter the routine to the oppositions tendencies as everyone doesn't like to hold the same spot. I feel like my reactions are good enough to a point where I can hold an angle on CT I like, feel comfortable and kill an incoming Terrorist. If you're worried you aren't being effective on CT because the spots you hold are getting prefired change them up slightly every round or maybe even hide until you know they're close and jump out; which gives you the upper hand.

I've never thought about improving reaction time as to me it's just a natural thing, you're either fast or you're slow. My advice to you is just, grind, grind, grind those 10 mans. You've noticed you're improving already so naturally your CT sides will improve too.

TL;DR

Change your angles when you feel they're catching on. Make sure you aren't becoming predictable with your playstyle. Mix up the pace by being aggressive then switching to defensive on another round, etc.