No, Zigbee is actually faster. Zigbee (and Thread) maxes out at 250 Kpbs. Recent Z-Wave generations max out at 100 Kpbs. Actual connection speeds may be negotiated lower. But pretty much either is sufficient for the low quantity of data sensors and actuators with these protocols tend to send, so it probably doesn't matter much in the real world.
Some would also disagree about the stability thing.

It does seem to depend on your particular devices. Xiaomi/Aqara has not always done well with Zigbee, using proprietary profiles that were
just close enough to Zigbee HA 1.2 that they could work most of the time with most coordinators but do odd things like "fall off" the network sometimes in many cases. I haven't seen reports like this with their Zigbee 3.0 devices (or with the hub they technically intended these to be used with).
I imagine Matter is one thing keeping them away from Z-Wave, besides history. Zigbee is very similar to Thread, and Thread has a pathway to "native" Matter support, which would not require a bridge, unlike Zigbee or Z-Wave. If they change to Thread in the future, Zigbee normally makes for an easier transition.