To use readCelcius from MATLAB you need to have used arduino() first. The function then places a spiread() call on the arduino object and manipulates the results.
The arduino() interface works by having a command monitor on the arduino side, and the MATLAB side sends a command to the arduino side, which the arduino side has to interpret and place the appropriate commands locally, and then the arduino side has to package the results and send them back, and the MATLAB side has to decode and return appropriate results.
This requires multiple encoding and decoding steps, and requires two or more trips across the USB/serial interface, which involves a packetized command-and-response protocol.
This is, of course, much slower than native code on the arduino that can make direct calls.
I would not expect the slowdown to be as high as you are observing, but it is going to be notably slower.
You can reduce the time by having a custom sketch on the arduino side that listens for requests (anything i/o will do) and runs the spi commands directly and decodes to a temperature that you send back. This would reduce encoding and decoding time but not bus time.
