That's perfectly fine. Don't worry about forgetting words. You will forget them, look them up again, forget them look them up again, eventually they'll stick. Focus on the reading. Don't treat it like a vocabulary lesson. Every day you're here to read, as long as you reach the end you're good, over months you'll realize you learned a lot of vocabs.

At first because the text will be so dense with new words yes it will take a long time to read, that's why I typically only read a short maybe half a page per day. Then gradually increase that as your vocabulary grows over months. The goal should be to encounter say 50-100 new words a day. Notice I said encounter not learn.

Those websites where you look up words are really useful. Make sure they have text to speech and read out loud in the language not in English even if you see the translation in English that's fine.

Also do a lot of listening along with the reading. I usually get myself an audio book and its corresponding text, chop it up into 1 minute and half a page segments, for each segment listen once, then read looking new words up, then listen while reading at the same time a few times, trying to follow a long, looking up any words I forgot, then listen without reading a dozen or so times until I can follow along. Then movd on to the next segment.