Oddly enough, I got the exact opposite reaction to 1803. I have a i7,1TB,16GB configuration with both the "System Protection" & BitLocker enabled. After the initial 1803 update my machine kept crashing / locking up, so I reloaded it deleting all partitions and started new. I partitioned my hard drive at a 60/40 split and BAM! - My machine is running super fast - It felt like some sort of 'break-in' period had ended and things started ticking, thus running like I would expect a 4.2GHz processor to perform. Yayyy
I had no problem with the 1803 update, but I also had wiped and reinstalled Windows a few months ago. I wonder if maybe something about installing fresh cleans up something that otherwise could slow it down?
I know some people tend to update the day they become available. I go a different route and traditionally wait 2 weeks to read up on how stable the update has been, based on commentary from those who have downloaded and been running off it.