For the most part I wouldn’t say serious but sometimes that can depend.
*These are rhetorical questions:
Is there a legitimate reason for the late reply?
How late is the reply?
What is the conversation at hand? (Is it an emergency?)
Has anything happened to deem a late reply suspicious or concerning? (Getting too close to someone else, Going through a rough patch in life, etc)
The thing is it's me who is reply late (they are too actually) and like they told me they were upset about me not replying fast (and I eventually explain why I reply late) it's either I'm too busy, too distracted(I have adhd) he's been replying to my messages super dry like barely effort as if he doesn't wanna talk to me for like 2-3weeks because I keep replying late apparently and I kept like yk apologizing etc and sometimes i ignore him because I'm in a foul mood which we talked abt. I explained to him that I would ignore him if I'm mad etc and I only do it when he gives me the reason to do so. What do y'all think? In the first place he was the one giving me reasons on why I reply late. When I want to talk like something to him he would say he's in game and that would make me mad etc and that results to late replying. I do eventually talk it out with him tho but lately he just keeps repeating the same replies. It's either "cool" or "ok" and then nothing more than that. He wouldn't even say goodnight or good morning or any I love you for the past 2-3 weeks. For me, I don't find it a big deal Abt him replying late but it was a big deal for him I did communicate tho saying I'll do better etc.
And my grammar can't even be correct due to how sleepy I am right now, honestly in our relationship I'm the only one who takes the lead in communicating. He told me he'd not be dry anymore etc because I was hella mad at that. I told my side and left a note saying I don't want any replies like "I'm sorry" "cool" or "ok" because I'm tired of it. It feels like he can't be bothered to put an effort to our conversation at all.

Do y'all think replying late to your lover is a serious issue or nah (any reason)