Yes, it’s operated by the 787-9. However, when United used to run LAX-SIN on a 787-9, it had to restrict seats. The flight is about 300 miles shorter than DEN-SIN, so they would need to restrict even more seats.
I’m assuming that this restriction led United to move its LAX-SIN frequency to SFO-SIN to avoid the restrictions.
PER-LHR I believe has wind advantageous, which decrease flying time and fuel consumption, but I’ll need to recheck on that.
Anecdotally, United was blocking 50+ on LAX-SIN during peak winter times… so with DEN, they’d have to block even more.
Truthfully, I’m unsure if there are any business ties between Denver and Singapore, or any ethnic populations.
I believe United also runs a slightly denser 787-9 configuration which adds a bit more weight to the plane, decreasing its range.