It's mostly because Garmin wants to maximize profits by sticking to old CPUs. The Coros watches (from what I've heard, the same applies to Suunto and Polar) are fast.
This has been an issue across the whole Garmin product line. E.g. the Garmin eTrex 32x from 2019 still used the same CPU as the eTrex 30 from 2011. 8 years without a CPU update. And the eTrex was already had miserably slow map rendering in 2011 with maps from that year.
> It's mostly because Garmin wants to maximize profits
I see people riding bikes worth tens of thousands regularly. They should try a top tier models and see what happens.
Although Coros watches are fast, the Coros DURA bike computer is pretty slow when re-rendering maps. Naturally, companies can still choose between how fast the CPU is versus how long the battery life is (100+ hours on the Coros DURA I believe).