Computing Rise Time on SPI Bus
Yes, the rise time will be dominated by the current available from the GPIO pin (it's wrong to think of it as a resistance) and the capacitance. The trace resistance can be ignored. 

Look at the guaranteed specs for rise time and what the load is for that guarantee. The capacitance of that output will be in addition to the specified external capacitance. For example, the PIC 16F series is generally specified with a 50pF external load. If your total capacitance exceeds 50pF, then the datasheet limits are not guaranteed. 

 

You must also take into account the stray capacitance from the traces. This can be relatively large if the traces are long, and the board is multilayer with ground planes or power planes close to the signal wires. 

Another potential issue is ringing, which can come into play at 8MHz. You may have to add some series resistance (perhaps tens of ohms) to tame ringing (especially on the clock), which will further slow the bus lines. It's good to hook a fast oscilloscope onto the lines and check the waveforms.