I download all the essays onto my iPad (Humanities - so between 2500-3000 words, and 8-10,000 word dissertations). Then I read through, and record my impressions as I read onto my iPhone, using the dictate mode in the Notes app, or straight into my laptop in Word, again using the dictation function (I don't touch type).
I then tidy up the feedback into generally 3 paragraphs - and not more than two-thirds of an A4 page, single-spaced.
1st par - overview & general impression
2nd par - strengths & weaknesses (with some examples)
3rd par - summary, what to do in their next piece of written work
When we used to get hard-copy essays, I annotated, a LOT - but I gather most students didn't look at our marginal notes, so this method is much quicker. Although it's a huge loss in detailed specific feedback to individual students, but heigh ho! Time is all ...
I'm a very experienced marker, but I still find it hard. It's a focused task that asks quite a lot of the examiner - it's often a self-examination about bias, clarity and so on. I find it takes me about the first 5 essays to get my head into the task, so I do ted to go back over them at the end as a sense check.
And you should be working with a 2nd marker as well, who can also offer an overview & further sense-check.
And our students regularly whinge about our personal prejudices. If only they knew how much we build in formal and informal guards against bias!