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), ....
Some further experimentation with my Sol'Ex. This time I'm using my ASI2600MC color camera. This camera has an APS/C sensor which results in a capturing a bigger spectrum. Getting a larger spectrum was easier to find the absorption lines: eg. CaK &H line, H beta, Mg triple, Na double and H-alpha. I was also able to find the slit, and thus the zero order.