i was busy that why replying late
i would like to thank you Joulinar
using separate power source for ssd solved my problem
im using raspberry pi 0 2w ,raspberry pi usb hub 3 ,raspberry pi 27 watt pd charger ,raspberry official 64 gb A2 memory card
first i was using seperate power source for both pi and usb hub ,but now im powering pi 0 from one of the usb ( since its usb 3 a port ,it gives 0.9 a at 5v which is more than enough for pi 02w )
my pi 0 running butter smooth ,no lag
it was indeed power source which was creating problem
now i dont even need cronjob ,my pi doest loose connection