This is normal for starting out reading.
I started out spending one to two hours per four panels, between looking up kanji one by one, looking up vocabulary, and looking up grammar.
Over time, you start to recognize the most common vocabulary and grammar (and you’re probably already there on common kanji), and the look-ups slowly subside little by little.
I did a post a while back on what one’s expectations should be when they first start reading:
I recommend that you reframe your reading experience as such:
Old frame: I spent an hour and only read through one page. This is disheartening.
New frame: I spent an hour learning vocabulary and grammar. I have made progress.
If an hour of study-via-deciphering-manga is exhausting, set half an hour as a target. You don’t need to complete a whole page in one session.
I was only able to properly learn these through exposure via reading, and even then it took me longer than it felt like it should.