I don't have access to a logic analyzer but my old oscilloscope is
almost reliable. I could confirm that the spi clock is indeed running at
the expected 25MHz, but I could observe some gaps of up to 320µs so
that's 8k spi cycles spent doing something else.
These gaps were observed on the SPI clock and the macphy interrupt was
active for the same ammount of time(though this was measured independently
and not on the same trigger).
I've been drinking way to much coffe, so soldering is not gonna happen
today (shaky hands), but if it helps I can solder wires to attach both
probes to confirm that the gap in the SPI clock happens at the same time
or not as the interrupt is active.

What i expect you will see is that the interrupt line goes active. A
while later there is an SPI bus transfer to fetch the interrupt
status, and when that transfer completes the interrupt should go
inactive. But you will need your second channel to confirm this.

One question would be, is 320µs reasonable for an interrupt, and one
SPI transfer?

I would then expect a number of back to back transfers, each around 64
bytes in size, as it gets the received frame. The gaps between those
transfers will be interesting.

	Andrew