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.
I see people riding bikes worth tens of thousands regularly. They should try a top tier models and see what happens.
I don't know if there are top tier models that run on replaceable batteries you can get at any gas station.