FreeTDS™ is not slow. We know this because we've tested it. It's measurably slower than the vendors' products for some operations, but it's not noticeably slower and it's certainly no laggard. If your experience is different, if you're waiting 30 seconds for simple operations or minutes instead of seconds for for query results, something is up with your setup. There are two likely culprits.
Logging. If everything seems a bit sluggish, check to make sure logging is turned off. TDSDUMP should not be defined, and there should be no dump file mentioned in freetds.conf. You can double-check by setting TDSDUMPCONFIG temporarily, which will log only the startup process.
DNS. If connecting to the server takes 30 seconds or 1 minute, you could do worse than to check your resolv.conf. Use host or nslookup to confirm that FreeTDS™ can actually resolve the name/address you provided in freetds.conf.
Packet size. The default packet size setting in freetds.conf (see initial block size) is usually fine. Slowness can potentially be due to multiple packet to use. Under GNU/Linux™ system we use an optimization to reduce network traffic; you shouldn't see much difference using this system.