I suspect your CPU is running only on a quarter, or less, of its performance.
You're saying,
- it's slow to get to the bootloader (with seconds count),
- it's slow when you're in Zorin,
- it's also slow with the rather lightweight Xubuntu,
Here are videos of Zorin on Celeron N4xxx, very slow, if not slowest CPUs around, listed in this specific post ( Disappointed with Zorin Core to save old PC (4 GB RAM way insufficient) - #26 by cc_spicuous @ September 20, 2025)
If you need something snappy and fast, I only know from the list of small distros that Puppy Linux is even smaller, between 300MB to 700MB, fits into RAM and is snappy on computers from the 2000s, just as live system. I think, it's good for using old hardware. Try the older, smaller 2019 version first. It's based on Ubuntu, so you can use command lines, like apt etc.
Puppy DLs
2019
Index of /puppylinux/puppy-bionic/bionicpup64/ (“Puppy Linux” 2019 @ September 5, 2025)
2025
Index of /puppylinux/puppy-bookwormpup/BookwormPup64/10.0.10/ (“Puppy Linux” 2025 @ September 10, 2025)