I'm using a regular Hard Disk of my PC to capture my astro images. For the Sun and Planets my camera's typicaly goes up and beyond 80fps. But after capturing for a few seconds, the frames per second drop. I always thought the rate would keep up in the back but I found out that this was not realy true.
This morning I did a test when capturing the Sun with ASI290MM:
When using an external SSD (Samsung SSD T7) the actual frame rate = 82,1097 fps
When using my existing Hard Disk on my PC, I start with 82 fps but my actual rate drops to 27.0601fps.
Conclusion: be aware that capturing time is depending on your set up eg. type of hard disk, connection (eg. USB2, 3), ....