They should in theory handle regular washing powder and a 40C long cycle in the washing machine, and it should not differ from a short cycle 20C. A longer cycle would get more dirt off. A lot of powders need time to disolve and during low temperature short cycles, the powder often ends up under the drum and get's pumped out with the water still in powder form.
I know a lot of "dry clean only" marked fabrics can be washed, but be a bit careful with temperature, color combinations, prints, and things like that. Shrinking can often be reduced by stretching when wet and ironing before it's completely dry. It depends upon your machine, but I would have maximum two pairs of curtains (90x90 cm) in the machine, maybe one pair at a time, lots of water, 40C, reasonably gentle drum movements, and letting them spend enough time in the soapy water.