Do the rates reset 1 month after release, or on the 1st of each month?
I was under the impression it was the latter, which really skews the results since you had no keys when the system was first introduced, meaning you've had two of the good-halves of a month rather than a full month.
I really hope you're right though. I got my first full key really quickly, and rates have been painfully slow since then. Also playing solo queue. I've completed exactly 4 keys as well. I've only received 3 key frags since April 1st despite playing a fair bit more than usual in that period. If they didn't reset on the 1st and are continuing from before then my rate would make a lot of sense. If it did reset on the 1st, my key drop rate has been WAY too low for a recent reset.
Don't get me wrong - I don't mind if they're slow. You need to earn it like everything else. I'm not going to max out my chests anyways, I don't even OWN 52 champions, let alone play them all at a level capable of earning S grades (although with mastery in featured game modes it may be easier since discovering an OP pick and playing it well on the first day before it becomes popular should get an easy S).