I experienced a similar problem, with constant disk writing at low rate (few hundred KB/s) and after inspecting the snapd process running on a container we found the following:

root@focal:/# snap changes                                                                                                                                                                
ID   Status  Spawn                     Ready  Summary                                                                                                                                     
234  Doing   2 days ago, at 16:40 UTC  -      Auto-refresh snap "snapd"

root@focal:/# snap change 234                 
Status  Spawn                     Ready                     Summary                          
Done    2 days ago, at 16:40 UTC  2 days ago, at 16:40 UTC  Ensure prerequisites for "snapd" are available                                                                                
Doing   2 days ago, at 16:40 UTC  -                         Download snap "snapd" (8008) from channel "latest/edge" (8.71%)                                                               
Do      2 days ago, at 16:40 UTC  -                         Fetch and check assertions for snap "snapd" (8008)                                                                            
Do      2 days ago, at 16:40 UTC  -                         Mount snap "snapd" (8008)        
Do      2 days ago, at 16:40 UTC  -                         Run pre-refresh hook of "snapd" snap if present                                                                               
Do      2 days ago, at 16:40 UTC  -                         Stop snap "snapd" services       
Do      2 days ago, at 16:40 UTC  -                         Remove aliases for snap "snapd"  
Do      2 days ago, at 16:40 UTC  -                         Make current revision for snap "snapd" unavailable                                                                            
Do      2 days ago, at 16:40 UTC  -                         Copy snap "snapd" data           
Do      2 days ago, at 16:40 UTC  -                         Setup snap "snapd" (8008) security profiles                                                                                   
Do      2 days ago, at 16:40 UTC  -                         Make snap "snapd" (8008) available to the system                                                                              
Do      2 days ago, at 16:40 UTC  -                         Automatically connect eligible plugs and slots of snap "snapd"                                                                
Do      2 days ago, at 16:40 UTC  -                         Set automatic aliases for snap "snapd"                                                                                        
Do      2 days ago, at 16:40 UTC  -                         Setup snap "snapd" aliases                                                                                                    
Do      2 days ago, at 16:40 UTC  -                         Run post-refresh hook of "snapd" snap if present                                                                              
Do      2 days ago, at 16:40 UTC  -                         Start snap "snapd" (8008) services                                                                                            
Do      2 days ago, at 16:40 UTC  -                         Remove data for snap "snapd" (7988)                                                                                           
Do      2 days ago, at 16:40 UTC  -                         Remove snap "snapd" (7988) from the system                                                                                    
Do      2 days ago, at 16:40 UTC  -                         Clean up "snapd" (8008) install                                                                                               
Do      2 days ago, at 16:40 UTC  -                         Run health check of "snapd" snap                                                                                              
Doing   2 days ago, at 16:40 UTC  -                         Consider re-refresh of "snapd"

The download is stalled at 8.71%. It's worth noting that I experienced connectivity issues two days ago and snap downloads were affected and they were very slow for a couple of hours before getting back to normal.