Longformer slow than Bert
When i set max_length = 2048, i found Longformer's speed is slower than commen bert, why? 
well BERT can only generate up to 512 tokens and both models are Autoencoding models and usually not used for causal language generation (Autoregressive models are used for this). You can check out the difference here: https://huggingface.co/transformers/model_summary.html.