With my Nikon Z7 which create 90M fits files (8256 x 5504) my download time is 1.92 sec to the internal eMMC storage (faster than a micro SD card). If I download to an attached SSD the download time is 1.84 sec.
I have the Z7 configured to capture in NEF (raw), store in FITS, capture target RAM, and save to the SD card. This is on a raspberry Pi clone (an Odroid-N2) which is a bit faster than a RPi4 (running on Ubuntu 20.04.3) built from the latest source. Maybe the Z7 is a bit faster in it's file saving? It uses a XQD card for storage.
If you are able to copy your system from your micro SD card to an external SSD drive, and boot from that, I think you'lll find your storage time improve.
I have a RPi4 with Stellarmate 1.6.1 (not recently updated) booting from an SSD drive, it also saves in 1.84 sec.