So slow it down to 10 events a second, if things suddenly dramatically improve then you know you have too much going on.
You have to send out 192s/device * 7 devices = 1344 bits of data.
Say the SPI clock was running at 1/4 speed, or 4 MHz. Then each bit takes 250uS x 1344 bits = 0.336mS
You are sending data out 20 times/second, or 50mS. So there's 49.6 mS for the code to do whatever it does, which seems like a lot of time.