update mints
set max_sol = (select max(lamports) from postbalances where account = mints.bc);

mints is a table with about 20k entries. postbalances is a table with 5 million entries.

The query has been running for an hour.
I assumed no lock contention since each mints row is being updated individually, and postbalances is only being read. However, the CPU is barely being used at all, so it seems like processes are waiting for each other.

I would assume that the CPU would be in use 100% to read/write data.

So what is going on?