ESP32 Bluetooth SPP Max Speed
This seems more like an overflow issue with something than an actual delay.  If you look at the time in the log, it seems you aren't skipping seconds in the log (aka, 19:12:32.234 in two sequential log entries when you claim a delay).  I think it's an error of working with larger than 8-bit data sizes.  I know this is an old question, but I was looking for info and came upon it and figured I'd ask.