When I first started doing ABC, we had nothing to work with with my ds. So for that, I literally sat there with blank paper writing ABC on three lines and filling in. Yes, it takes a long time! However now that we have more data, we know his typical responses. So now we have really detailed forms we use that allow us to gather data very very quickly. When I'm making data or the worker is recording data, she typically just uses the sheet for 10 minutes and boom is done. That's for a 3 hour session. Yes we're having less behaviors now, but also it's that the form is easy to use.
Some things don't need ABC, but that's like how many times did he X. So even though we ABC major things, we also want data on those smaller behaviors that you just tally (hitting, pushing, licking, grabbing, etc. etc.).
I would encourage yourself that this is a stage, that she'll probably make some forms for you to make it easier very soon. It's awful now because you're beginning and because he's having the behaviors. Not fun. I was filling 1-2 pages a day trying to ABC, sigh. Not a fun stage. But now we just use a form for each 3 hour session and boom. You'll get there.
What I did to tide me over was I would make notes on my phone or paper during the session. Then I would take a break, 2-3 hours in, whatever I needed, and I would have him do something independent while I did my ABC. It was still fresh in my mind and the notes jogged me enough.
You're probably going to notice a more complex flow, because you have ABC and then he has a RESPONSE to your C and you get ANOTHER ABC kinda gig going, lol. Or what we do is we mark response to the C and duration. So our ABC chart is actually 5 columns, with the most common things typed in with boxes to check. Makes it very fast. But you have to make some data for a while so she can make that for you.