The larger the payload included in each frame, the smaller the relative cost of transmitting the header and trailer bits.
This is a noticeable improvement over the fixed 25% overhead of start and stop bits required by direct asynchronous serial communication, but obviously such large frames impose a cost in the form of communication latency.
LAPM has an optional selective reject (SREJ) functionality which allows it to resend only the corrupted frames, providing faster recovery from an error.
The ITU-T V.42 LAPM procedure is considered more robust than the Microcom Networking Protocols (MNP) that preceded it, and has a more sophisticated and powerful data compression option in V.42bis, allowing much greater data throughput.
Even while in uncompressed mode, the modem monitors the characteristics of the data and switches compression back on as soon as some benefit can be achieved.