I believe the ESP8266/ESP32 hardware SPI code isn't that speedy in general. Espruino expects to be able to queue a byte to send, then go away and work out the next byte while the first is sending. The ESPxx API doesn't let it do that so it has to send a byte, pause while it works out the next one, and so on.
You could try software SPI? Not sure if that'll be faster, but it might be as it runs as fast as it can - especially if you don't specify a MISO pin, because you don't actually care about receiving data.
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