@Tom-sl Your innodb_io_capacity is tuned too high. Please lower that to 400. Also, userstat=1 can CPU intensive, you should disable that too. You only have 4 vCPUs so wsrep_slave_threads should not be larger than 4. Additionally, 16 buffer pool instances for only 10GB of buffer pool is too many. I would reduce that down to 2 instances. Lastly, I would increase innodb_log_file_size to 1G You can make those changes and simply restart MySQL to take affect.
Are all of your tables using the InnoDB engine?
Check PMM and look for various things like Disk IO maxing out, or CPU saturation maxing a single core. Look at the replication graph, are you seeing large spikes in lag? You can send a few screenshots here.
It’s very difficult to diagnose an issue like this over forums, but I’ll try to help out.