Delays are normal if the block is geting bigger, but that has a point in Bitcoin where the 1M limit is almost always full.

In Zcash, 20 or 100 K is not so big to cause such delays.

I suspect that has to do with the shielded transactions, they somehow "consume" a lot of resources in the block.  If you see inside the latest blocks  (https://explorer.zcha.in/blocks) they are many of them.

The Shielded Value Distribution is increasing, 40800 Zecs:  https://explorer.zcha.in/statistics/value , which is nice of course.  It will have a positive impact on the price.