If your messages always sit on read while theirs get instant replies from you, the math is telling you something.

Effort leaves a record, and the record is more honest than how a friendship feels from inside. If the responsiveness runs one way for long enough, that is not a series of coincidences, it is the shape of the thing. Seeing it plainly is uncomfortable, because you have probably been explaining it away for a while.

Try matching their pace for a few weeks rather than sending an accusation. Notice whether they close the gap or whether the contact simply stops. Either result tells you what you needed, and neither one requires you to think badly of them.