How to read old iMessages on Mac or iPhone?

Hi, I have iMessage backup on iCloud from my 5 year.

How to read iMessages older than two, three years on my Mac for example?

Where is problem? When I scroll messages from last weeks or months, the system download this messages quickly. The further we move back, this download speed decreases. After few days I am on three years back and now every message need few minutes to download.

How to deal with this? Why such a backup if you can't back to old messages?