A common approach is to use multiple electron beams in parallel ([1] is up to 262144 beams!). This is starting to be used commercially to create the masks for photolithography.
AFAIK, the best performance IMS were able to achieve with the 512x512x50nm e-beams was 1cm^2 per hour. That's acceptable for etching masks, but still not feasible for chip manufacturing, as wafer goes through 40-50 exposures each would be taking days to complete.
[1] https://www.ims.co.at/en/products/