If, and that’s a big IF, we were all being honest then we could probably all do 95-99% of our current speed on a Decathlon bike…
In my experience they’re pretty well built (certainly more built for use than looks) and would be my go-to place if I decided on a new “new” winter bike (I mostly buy 2nd hand though).
We’ve had a few kids bike from there which have been mashed more than any adult bike would. They still run really well with minimum maintenance. It’s also stuff such as fully(!) covered chains and other proper usable stuff that they do, where even Islabikes fall short and seem to go for looks or “fast and comfortable long distance bike for your 3 year old”. I’ve always found Decathlon stuff to be situation aware with price and equipment.
Decathlon bikes are generally heavier but it seems to come from well placed sturdiness rather than simply cheap stuff.
So that all sounds like an advert, but I actually really do believe it.