Hello!
If you have the latest firmware installed, then take a look at your sd card, maybe the lack of power is there.
I bought a PNY with 100Mb/s and experienced a big improvement in photo write speed, compared with my older SanDisk 30Mb/s
By the way, I believe if you use a "slow" card in a brand new X-T1 probably you would be very upset with the performance.
Cheers, ...Edupi