Has support escalating this within engineering? We sometimes see degradation/bugs around new releases but your issues seem a excessive.
How large are the smart campaigns you're running?
One thing I've started to do for larger batches is:
1) Create a smart list
2) Create a static list
3) Run a smart campaign to batch the smart list leads into a static list (this may help as you mentioned above some of your smart lists reference multiple static lists)
4) Run the flow actions with only the "member of list" in the smart campaign filters
It's a bit of a pain up front, but ensures large email sends are run promptly. I'm sure the thought of sending an email without a count PRIOR to launching is terrifying!
To Caroline's point, definitely looking forward to the performance improvements rolling out the forthcoming months!!