astropy: Time creation for unix time is very slow

This issue is very likely related to #5846 and #10167.

When using astropy.time.Time to convert unix timestamps (or utime) to datetime, it’s more than factor 1000 slower than using datetime.utcfromtimestamp().

I benchmarked converting 2000 values, which is not even much when reading fits files

Time(times[i], format="unix")  # 1.61s
datetime.utcfromtimestamp(times[i])  # 998µs

Time(times[i], format="utime")  # 1.74s
datetime.utcfromtimestamp(times[i] + offset)  # 2ms, offset being set to Time(0, format="utime").to_value("unix")

Astropy does some special checks for leap seconds and so on, but I haven’t found an example where the native conversion differs from astropy using seconds as the smallest unit… is there any at all? If not, would it be feasible to convert those two formats using the native datetime module? If not, what is causing this huge difference…? seems a bit strange to me