I've been wondering if the temperature in the water reservoir would have any effect on rebound time or even shot temp-stability in a HX machine? Long story short, I refill my reservoir about every morning, with 25% tap, 75% RO water. The RO water comes from a dispenser that refrigerates the water pretty significantly. So would this cause my machine to rebound more slowly after the shot? I'm using a QM Anita.

Thanks!

Unless you are filling the reservoir with ice water, completely, it is not something to worry about. The ambient temperature of the inlet water will have an impact but it is minimal, a matter of seconds most of the time.

When I had Rocket Espresso HX machines, I added chilled water from my refrigerator water dispenser each morning, and as the machine warmed up prior to use the water in the reservoir was warmed up too.

Rocky

the only check I've done, was with my previous Bezzera Magica (E61, HX) when I doubled the ss wall between the water tank and the boiler space, with some thermal insulation material;
definitely, the result was a less stabile machine! and I was using a temperature data logger with 2 probes - like I'm doing now, with Strega;
so I remained with the impression that in a carefully designed machine, the thermal equilibrium is considered with the tank filled and with a slightly warmed (30C) water; the water which will be injected into the HX; and the status (the "full" idle) is a reference one;
it's not only about the recovery time to the next shot, but it is about the whole thermal mass of the machine; larger, the better;
well, in a classical design, IMO...