Getting really slow page loads, 500 error on first login attempt. Contact host support, this is a server issue.
You're currently running in php 7.2.19, logging a lot of errors (currently 16,506,635 in log, you're logging over a half million per day). Your _log_errors table in database must be enormous. SMF 2.0.15 supports up to php 7.1, you need to downgrade to 7.1 in cpanel (I no longer have access). Don't think that's causing lag, but doesn't help.