I tried both Zero and Zero(w) with the same result.
I also tried "overclocking" CPU AND SD card but I didn't use a "fast" SD card(Samsung or Scandisk 99mb transfer rates) - just a std cheap SD card but still,IMHO,too slow when Ekos was used!
Without Kstars/Ekos but GUI running it was just usable even over VNC but no Astro stuff - except for the WiFi drop outs (there was a bug on early Zero(w) I believe ).
Extra memory always helps so 512mb is the limiting factor I guess - didn't do any bench marking as it was a "i winder if it will" moment.