1. Your PHP's capability in handling MySQL is old compared to what your current installed MySQL can perform. This can be compared to putting a racecar driver in the seat of a spaceship, some things he will do just fine, certain other things he might not know how to do at all.
Suggestively you should update your Xampp version. (Or update your PHP manually if you know how to do that)
2. Optimizing the database can be compared to windows defragmentation utility, it reorders the data inside of your database to allow best read/write speed.
Its up to you what you want to do, its only really useful if you got alot of data in your database (Meaning, lots of players, items, accounts, etc...), but on the other hand it only delays the startup by a couple of seconds.
My suggestion is, turn it off until your server gets a few more players, something like 20+ players online or 50+ players in your database, then you can turn it on again, that way you save yourself a few seconds of startup.
3. The version of xampp itself does not matter in terms of OT servers, since it's not a requirement per say. You can run an ot server without it.
You only need XAMPP if you want to have website-based account management available, and this only if you do not know how to manually set up mysql/php/apache.
Again even if you do want to have web-based account management, the version of XAMPP does not matter, as long as it's not 10000 years old or something, then you might have problems with that the AAC requiring certain functions that your old PHP cannot offer.
But it's always recommended to run the latest version of any software, since as long as there is software, there will be bugs, glitches and exploits which is being constantly patched in new updated versions, this especially applies to XAMPP since it is in fact a software PACKAGE containing 3 different softwares, each one being patched and updated regularly.
