Some of the cheap HTs, which were designed for voice, key up the transmitter quite slowly, and the transmitter also stays on for quite a while after the PTT is released. At least my older Puxings behave this way - they require a long txdelay in the tracker (delay from PTT down to the start of data transmission), and there's also a long "tail" after the data.
It might have to do with power saving methods, which can be disabled in radios which were designed for data (like the ham rigs with built-in APRS).
You can compare transmitter key-up/key-down timings by attaching another receiver to a computer, opening the squelch completely, and recording the APRS transmissions using a wave editor such as Audacity (free). Zoom in to the start of the transmission - it'll be quite easy to see where the txdelay flags end and actual data starts. If your tracker has a txdelay setting of 20 (200 ms - they're traditionally set in 10s of milliseconds), and you're seeing 50 ms of flags before data, the transmitter takes 150 ms to transmit. If you're seeing 150 ms of flags before data, the transmitter only takes 50 ms to wake up. On some of these SDR-based HTs the delay varies a lot.
Zoom in to the end of the transmission and see how long the transmitter transmits after the end of data, too. Some stay on longer, wasting more bandwidth than others for the same amount of transmitted data.