Old All In One Speeding up

Not big expect but have replaced HDD and RAM in HP and performance increased
Ps – mine laptop also 6 year old
Working smoothly without any lag only issue battery now last for just 1.5hrs on full charge

replace HDD with SSD and feel the difference yourself !
no doubts

1st of all, why are u speeding up your laptop? If u want to play games or want to do heavy tasks like video editing etc then please buy a new laptop or a desktop(preferably) but if u want to speed up booting time and want to do simple tasks like study, browsing, office work then just replace you hdd with ssd and extend your RAM as other dimers suggested u. I also done the same with my 10 years old pc 2 years back and it’s good to me. You will really feel the difference. Nothing extra you have to upgrade rather than those two i have mentioned above.

use ubuntu

Ssd helps in booting speed and app load time along with file copy paste speed. But you’ll have to reinstall the windows and all the applications.
Additionally u can add same amount of additional ram if your laptop has extra port. But i would suggest to invest in ram only if the ram is of such configuration that may be useful to u or somebody else in future else u wont be able to use it again if in future u change laptop, ssd u can use in any computer

Replace Windows with LXLE or XUBUNTU (both based on ubuntu)
Replace HDD with SSD
Increase RAM

You need to give a lot more info like year and model number etc..

Adding an SSD would speed things up a lot due to small hard drives being very slow and getting de-fragmented files. Adding extra memory would speed up with large bloated programs and can also be used as a cache by the system. I used to be on an I3 sandy bridge which was from 2011.. Speed around 3.2GHZ.. When I replaced it with a newer 3.7GHZ CPU, the change was very noticeable.. Everything else being the same, things were done much quicker.. Even though only 1 core of the CPU was mainly used, it would do it much faster.. Usually you need double the clock rates to notice changes and now with 8 core and higher CPU’s, it still makes a difference what the top speed of the CPU is. its better to get a cheaper 2 core CPU than a higher priced lower cloaked 4 core CPU for most people. The more cores the larger more complex applications you can run but how many email and web windows would you have open anyway?

First try to find out the bottle neck, before changing ssd/ram/cpu.
You can find it out after re-starting the laptop and then check the Task bar. Mostly, it will be ssd/ram/cpu in that order.

I replaced the HDD with SSD, keeping everything same. I did not see a noticeable performance improvement.