Think of it this way, how long does it take for your PC to boot up? New TVs are basically specialized computers. Back in the old days you would have to wait for the tubes to warm up, now you have to wait for the processor to boot.
Basically they are loading software when you start them up. Some TVs use Linux (with busybox) and that takes some time to load. I know for a fact that Sony uses it. In fact you can download some of the executables and code from their website: Sony - Open Source Code - English
LG also makes busybox source code and all of the other GPL and LGPL code they use available on request.
I purchased a new Sony recently and everytime it was switched on as well as the Sony logo appearing it also produced the Sony sound jingle. The menu options offered no way to disable the start up sound and it was volume control independent.
I ended up have to download a firmware update from Sony and install it on the TV. I now have the menu option to disable the Sony jungle sound on start up.