There are a lot of variables that determine how long it takes to "launch" the program. The model has a lot to do with it. If you have an older 3G, it takes longer than a iPhone 4. If the iOS needs to 'free' up memory, it must do that while it's launching TM. On my older 3G, sometimes it would take 5 seconds (with over 1000 records) and sometimes it would take 12 seconds. With a 3GS or newer, it may take several seconds on the first launch when you have iOS 4+, but after it is "loaded" it only take 1 second to launch (multi-tasking).
If you are really curious, you can always make a Backup with Time Master Central (or the iTunes "File Sharing - Documents", do not rely on iTunes itself for backup). Purge all your old entries and see how long it takes. I believe that once you do this, you will find that the difference is very small. You could then Restore your data back again. Just make sure you Restore before entering any new data!