In all honesty though I haven't a clue what's going on mate. I don't even know what USB Legacy or Windows Boot Mode even is. From a quick look around on the interwebs, I found people saying they had problems with this USB Legacy stuff and those problems went away when they removed all connected USB devices. If that doesn't help, maybe try (re)installing the latest BIOS of your computer?

What are the differences between Ubuntu and Scientific Linux?

Ubuntu is like OSX, it restricts access to a lot of stuff that shouldn't be restricted. Scientific Linux is a clone of RHEL and is more enterprise-oriented, so you won't get all the bling (by default), but you will get easy access to whatever you want, you get stability and support for about 10 years IIRC.

Gotta go now though, I'll be back later tonight or tomorrow to give a proper answer, but now: If Anything Happens To The Cat!

The fast or slow BIOS loading is  unrelated to the system installed. I would like you to tell us what kind of BIOS do you have and check the following settings:

1. Memory Check on boot (lots of slow-down before the BIOS floods the RAM fully, may be disabled)
2. Boot priority. Enable the drive the system is on, disable or move down the list all other devices in order of importance.
3. Graphics Chip Selection. If you are not using the on-board chip, set it to PCI-E rather than auto.
4. SATA/IDE Drive Configuration. Detect the settings and save them rather than keeping them at Auto.
5. Memory Settings. Do the same as above - detect and save the settings rather than leaving them on auto-detect.
6. Overclocking. Return the values to stock ones if they are not default. In the event of a too fast frequency setting, CMOS detects the default one and uses it, often without notifying the user and slowing booting down.

If the issues remain, try using Fail-Safe Settings or return the mainboard to Factory Settings. If that does not help, send me photos of each and every BIOS menu there is.

Also, make sure it is the BIOS that causes boot lag - if you mean the bootloader then it's a different issue entirely.

I once pressed the "sleep" button on my keyboard by mistake.
PC went in sleep mode but it did not "wake up" anymore, no matter what I did.
It was a BIOS issue.
So I simply removed that little battery on the motherboard and then placed it back in.
Afterwards the computer started booted up normally, the only difference being that it made a rollback to an old BIOS version.
So I simply installed the latest update and everything was fixed.

Don't know how good it is to remove that battery thingy but it worked for me.

Thanks Foxi, I will check those things. But if it didn't help: How am I supposed to make pics of my BIOS menu?
And I'm not sure if my BIOS is that cuases bootlag. It's just weird that my computer takes forever to boot.

With a phone, seeing that you can't take screenshots.

If the BIOS is fine and you have more than one system, install a custom bootloader. If you have just one system then defragment the drive - it might be fragmented.