PPP throughput gets throttled when atService is used in Legato in parallel with PPP session



When we have a PPP session open with a PC (WP76xx) over /dev/ttyUSB2, the data throughput is about 30mbps as expected.
Then we run our application (reading AT!LTEINFO) using the Legato atService (/dev/ttyAT) and the throughput gets throttled down as the AT command interval goes from ~55ms to ~5ms.

Q1. why does the PPP traffic on /dev/ttyUSB2 get effected by AT commands on /devttyAT?
Q2. If we use the QMI interface instead of /dev/ttyUSB2 (PPP) then how does it work and will a QMI data session be effected ? Where can i find more info on QMI and how to use it properly?