Just give XP some time and it will slow down while Ubuntu will run the same.
Linux does not slow down as you add applications because applications do not
run from the system tray and stay resident in memory. Linux does not require
anti-virus, anti-malware and anti-trojan software running in the background.
Linux does not require re-booting so that you lose productivity. Linux has
no registry that can become fragmented, corrupted and balloon in size. Linux
file systems do not become fragmented as NTFS does. What you get with Ubuntu
will be the same throughout while Windows performance degrades over time.

How fast something runs is often a matter of perception rather than factual.
Many Windows users cut Windows a lot of slack and never seem to notice its
faults. For example, they just assume that re-booting is th enorm and don't
see this as down time and therefore don't factor it in. They do not factor
in the lost time it takes before you can use your computer with all of the
updates to ant-virus and other software. On my wife's Windows laptop it is
good five minutes before she can start to work as things load into memory
and begin updates. I can boot into Ubuntu in less time and be ready to work
right away and never be nagged or forced to re-boot.

Finally I would add that XP is older and does not come with as much built
into it. Ubuntu as grown in size over the past several of years and is
slower when compared against past versions of itself. Windows XP has had
only three SP upgrades. Ubuntu supports more devices than XP. You need to
add drivers by inserting disks. Ubuntu comes with them. The Linux kernel has
grown in size because it provides support for many more devices. To get my
Wacom Bamboo tablet working in Linux I just need to boot the computer. To
get it working in XP I need to install a driver and then I must re-boot.
When I plug it in it takes time before that driver is enabled. In Ubuntu it
is immediate. Ubuntu comes with compositing, XP doesn't. Ubuntu supports
Windows file types, XP does not support Linux file systems. I could go on
and on about the differences. They are many and varied.

Users have a choice. The systems are radically different. You are comparing
apples to oranges. If you prefer XP then use it. Many Windows users prefer
XP to Windows 7 or Vista for the same reasons you mention when comparing it
to Ubuntu.