I've had the same slow read time issues from the day I bought it. The fact that I could replace the probe was a big selling point to me. After having some experience using a meter and this one in particular, I'll definitely be looking at some other models when/if I ever have to replace it.
How fast a pH reading stabilizes is affected by a variety of factors. They include:
1) Probe properly hydrated
2) Is the probe clean
3) The type of junction used
4) Is the junction clogged
5) The conductivity of the solution
6) Age of the probe
If a probe is not hydrated then as the probe sits in a solution a thin hydrated layer will form on the glass. This change will affect the electrode characteristic causing the the calibration not to be valid. Overall there will be a very very slight drift in readings. For this reason it is important to store in a solution. Whether storage (ideal), 3.5M KCl adjusted to pH 4, or buffer solution all will help to maintain the hydration layer that takes about 3-4 hours to fully form.
Even though hydration can be an issue, a coating on the probe would more likely be a source of drifting than the lack of the hydration layer. Every pH probe has two wires. One is inside the indicating glass electrode and the other in the reference. If a coating is on the glass it will impede the circuit causing stability issues.
The other are of concern would be the junction. There are many different types of junctions including those made of ceramic, cloth, Teflon, glass, and open. If the junction gets clogged the circuit is impeded and erratic readings will be seen. The Milwaukee pH-56 has cloth junction. it is about 1/8 to 1/4" wide and about 1-2" in length. If you look at the bottom of the probe you will see what looks like a small piece of paper sticking out. It can be seen on the link to the Milwaukee site and the Hanna site below. The junction of the pH-56 is extractable. When erratic readings are observed simply pull about 1/8" of the junction out. This will clear any clogging and the response time should change dramatically.
All pH electrodes need a solution that is conductive to work. Typically at least 100 uS/cm. Mash and Wort should be well above this so it should not be an issue. 100 uS/cm is pretty low. For example, if I remember right, Lake Michigan is about 300 uS/cm. Reverse Osmosis water is 30-40 uS/cm.
All pH electrodes age. The glass does breakdown. A probe should last at least 1-2 years. One of the things that cause probes to fail prematurely is using at high temps. We typically say at 25 oC (ambient) a probe will last 1-2 years. Every 25 oC increase cuts that in half. So at 50 oC a probe will last 6-12months, at 75 oC 3-6 months and at 100 oC about a month. As the resistance of the glass changes so does the impedance of the circuit.
There different glass types including those that are made for taking temperatures at higher temperatures. As it was pointed out earlier there is the argument whether the pH value at one temperature is the same at a different temperature. The ATC of any pH meter is correcting for the change of the resistance of the glass with temperature. It does not compensate for the actual effect of temperature on any particular solution. Considering that unless you know that you have a good offset/slope the actual reading obtained with any meter is going to be less accurate than the affect of temperature on the solution. I have seen 0.5 to 1 pH discrepancies. I usually see this when a probe is being used well outside a good operating spec of +/- 25 mV in pH 7 and a slope greater than 90%.
http://www.milwaukeetesters.com/pdf/product-specs/pH55-56.pdf http://shop.hannainst.com/hi98107-phep-ph-tester.html