The Transients API is similar to the Options API and that means as of right now, unless they make a change in v4.1 or the future release, all of your transients are going to be moving in and out of the
tableprefix_options table. If your website is busy, that means your going to have a ton of reading & writing taking place which defeats the purpose of caching in the first place.
As it stands, there are no hooks being made available that will alter the function and only three options are accepted:
- Name of Transient
- Value of Transient
- Expiration of Transient
If you were to write your own code (ill-advised in this case), your option would be to hack core, which is going against best practices and you'd be on your own for support as any issues arising from such would result in the community (mods and other users) advising you to revert back and see if the issue persists.
That being said, my advice would be to make use of the options outside of WordPress and combine them with what is available to WordPress by way of a plugin. This means enabling Query Caching through MySQL/MariaDB/Percona/etc, using Memcached or Redis etc (Redis wins over Memcached unless you're storing extremely small bits and pieces). This way you have a data store that isn't going to hammer one specific table over and over again.
W3 Total Cache supports APC & Memcached for Object & Database Caching (as well as File Caching, but it's often not recommended, especially if you have super-slow disks - i.e. anything other than SSD's), but it's bulky and likely overkill as the vast majority of the options can be done at the server level (i.e. file caching, browser caching etc). For Redis, you can use WP Redis and it'll handle object caching for you and it's to the point. If you've never used Redis, I'd check out:
redis.io (can't post another link else I'd of linked it for you :))
To enable Query Caching within MySQL, you should simply need to make sure the following exists in your my.cnf (normally /etc/my.cnf or /etc/mysql/my.cnf depending on OS):
query_cache_size = TOTALCACHESIZE
query_cache_type = 1
query_cache_limit = LIMITSIZEOFRESULT
Where TOTALCACHESIZE is the total size of the query cache as a whole and LIMITSIZEOFRESULT is the maximum result size that will be cached. Anything over this size will be ignored. You can specify the sizes in K OR M (no B on the end) like so (this would be a general starting point):
query_cache_limit = 512K
query_cache_size = 16M
query_cache_type = 1
NOTE: BEFORE dropping that in, shut down MySQL, backup your my.cnf file and then make the changes.
If you make changes while MySQL is running and then restart/reload in to the new config, it is possible you're get an error stating that MySQL failed to properly shut down and you'll then have to troubleshoot (it's happened to me a few times).
Beyond that, there are other ways to speed up MySQL by modifying the configuration, but it's too easy to get carried away, so starting with one of the easiest methods to speed things up while increasing performance is the best route.