"if there is lots of physical disk space available ..."
As your database is set to Autogrowth then it should automatically extend as needed. Perhaps make a note of the size of the MDF and LDF files today, and check again in a few days time and see if it has grown.
I'd also suggest changing 10% to some suitable size in MB - the 10% becomes a BIG extension once the database reaches several GB in size, and can impact on performance badly on a busy database.
"data access has gone very slow"
Make sure you are running UPDATE STATISTICS regurlarly (and perhaps to a less extent using REINDEX or INDEXDEFRAG maintenance commands.