I work for a Law firm and each database is a case we are representing. Total we have 10+ TB of data (mdf + ldf size). Initially I thought the slowness was because of our increasing number of cases, but once I reboot the server it runs and the transaction log deletes at normal speeds (within 5 minutes). But then over months in slowly starts to slow down in regards to deleting the logs.

Performance is normal and values are always within acceptable baseline ranges, even when the deletes are slow.

Like mentioned using Forfiles (via TSQL job) always deletes the files without issue and super fast. It is only when using the maintenance plan and deleting .trn files that the slowness begins to occur.