Thanks
I'll need to write some totally standalone code and do the timings again, but I'm not running any interrupts, so I can't see how that would make much difference.
The SPIFFS file are written using code, i.e the code downloads the command files and stores them in spiffs, so I suppose this could be related to how the file was written, but I'm pretty sure I had slow file access when I was using the SPIFFS as a web server from files uploaded using mkspiffs and the esptool
I'll need to do more testing
PS. I'm still getting intermittent crashes, which only seem to have happened when I stated to use SPIFFS in the code, but I've also added the use of the Time class (NTP), so its possible it is doing something unexpected