According to the survey by Dennis and Zarro (Solar Phys. 146, 177, 1993), the Neupert effect holds for slow events as well as fast ones. These results confirm that. We have correlated the BATSE fluence and GOES peak flux as shown below:
The dotted line marks proportionality and is not a fit to the data. This correlation is not beautiful but it is real: all "slow LDEs" have weak but non-thermal hard X-ray emission.