Adult system (a1) is expiriencing issues because queries that are meant to be executed on slave are (for yet unidentified reason) returning empty results and falling back to master. This causes slowdown on sites (but everything is operational).
EDIT: after a dozen of changes where we've tried to offload work from master as much as possible - situation is looking stable. Response times are back to normal. Our changes might have helped, it's still unclear how situation got sorted, we'll be investigating further and continue to work on improvements.