If you never defrag the MFT or shrink and defrag registry hives or clean up winsxs then it will inevitably slow down.
If you clean up old log and temp files, purge outdated/orphaned registry entries, use a registry hive optimizer and defragger, and defrag the MFT, any slowdown will be very slight. I have had Windows 7 installed on my Precision M6400 laptop for a lot of years now, cloned it from the original hard drives to newer hybrid hard drives, and have not experienced this slowdown.
Tools I use:
* a batch file I wrote to clean up what ccleaner misses
* registry life
No problems at all.
I'm planning to upgrade both hard drives to SSDs in the next couple of months... I'm waiting to see if a new higher resolution 17" Precision Mobile Workstation will be announced in the wake of new video chipsets. If they can't do higher than WUXGA or support at least three screens, I'm upgrading the M6400 because it is still plenty fast for my work, and for photo editing on the go. I run a dual boot configuration, Linux for work (BYOD rocks!) and Windows for play and hobbies.
I'd love to just punt Windows... but Adobe CC, embroidery software for my embroidery machine, and some of the games I play are unavailable for Linux, and wine and derivatives (including crossover) is too incomplete to be an adequate substitute. Plus, Netflix: sure I can mess with Moonlight for hours, but why bother when I have the Windows license that came with the laptop and I can just reboot to Windows to watch streaming video?