There could be any of a number of causes, some fixable, some not. Here are a few that occur to me are:
1. The Pi0/Pi0W are among the slowest versions of the Pi. Check the settings to verify that it is running at 1GHz, as I've seen out-of-box installs run at 700MHz (the original default speed for the BCM2835 SoC).
2. Your SD card may not be very fast. This can be fixed by copying or rebuilding your SD card on a faster model. I'm told that the best current ones have a "A1" rating.
3. What else is your Pi doing? Are you running any other programs concurrently? The Pi0/Pi0W has a single CPU core. So while it has algorithms to share the available processor cycles among the tasks trying to run, it can really only do one thing at a time.
If you can replace your Pi0/Pi0W with a Pi3B+ you should see about a 10 fold increase in performance.