Two GPS positions need to be used to calculate speed and then the calculated information can be rendered on the device. As most built in devices receive GPS data about once per second it can take up to a couple of seconds to calculate and display the speed.
The app processes the GPS data as soon as it is received but without clever predictive algorithms the delay is as small as I can make it.
I’ve recently been using an external Bluetooth GPS receiver that can send updates at up to 10 times per second – about 10 times faster than the receiver built in to phones and tablets. I’ve found that the lag is eliminated with this device and DigiHUD works perfectly fine with it. I’ve tested it with three devices – HTC wildfire (Android 2.3), Samsung Galaxy S3 (Android 4.4) and an LG G3 (Android 5.0) and each one has performed great with no issues.
More info on the Garmin GLO Bluetooth GPS Receiver.