Unless you have a washer dryer, why can't you overlap washing and drying cycles? Surely that would take 4 hours because load 2 and 3 can be washing while loads 1 and 2 are drying.
This is definitely an advantage of a faster dryer. I went from using airers which could hold a maximum of 2.5 loads of clothing which would then take about 48h to dry, to having a heat pump dryer which can dry a load in about 90 mins, so I can get through three loads of washing now in about 4.5-5 hours which is definitely an improvement esp when we have a lot to do at once like after holidays or when someone has a sickness bug. An hour would be good but 90 mins is already miraculous enough for me.
I don't know why some machines take 6 hours to do a single wash and dry cycle but it seems like either it's on a very slow eco setting, or something is wrong. I know for example, my washing machine sometimes uses its "AI" and thinks if you've put something in it wet, that you've actually crammed about 20kg worth of dry textiles in somehow, and will schedule some kind of heavy duty 4 hour wash. When we got it, the old machine had died in the middle of a wash cycle and so everything was absolutely saturated. I assumed I could just shove the wet things into the new machine and it would cope but no, it was trying to be clever. If I'd based my entire opinion of the machine on this first impression it would not have been a good one. Now I know how to use it and what it's trying to do it makes more sense.
It turned out there is actually no way to deal with this other than to trick it by taking everything out, putting in a smaller amount, letting it calculate based on that and then pause and add the extra stuff. The website is spectacularly unhelpful basically saying "don't wash wet clothing then" - I mean seriously? I do sometimes feel like designers of these products don't seem to have ever actually used one on a day to day basis!
I know for most washer dryers, the drying capacity is much lower than the washing capacity which people don't always realise, but also I feel like the manufacturers don't make it clear either.
I won't even get into the dishwasher I had which had a stupidly designed multi gear sprayer which doesn't come apart for cleaning, and so would constantly get blocked up with tiny bits of food debris and then shoot high pressure water at the door seal which would overwhelm it, causing leaks. The enraging calls to customer service trying to explain that their product was poorly designed. It managed to kill itself by jetting water into the electrics so we replaced it with something without that issue. I do wish there was a better way to share experiences and read reviews though because there always seems to be so many variations on every model of appliance that it's impossible to find useful information about any of them.