This is specifically for the DS18B20 but probably true for other thermometer parts.
1. The fastest way to read a temperature at a particular resolution is to set the default resolution to that and then just do a owread temperature rather than owread temperature9 ... temperature12. What those do is to set the resolution each time. It would be useful to have a set resolution option in OWDevice to support this method.
2. The fastest way to read multiple thermometers is write 1 to simultaneous which (I think) issues a skip ROM then convert (so tells all to convert). I assume a Convert T instruction (44h) is only recognised by thermometers and other 1-wire devices on a bus will ignore it (if you have other devices). Again it could be useful for OWDevice to support this. You can owread latesttemp which skips triggering a conversion. All devices converting at the same time will give a higher peak current of course. DS18B20 is 1.5mA max active/converting and 1uA max when not.