Why doesn't an SSD improve the speed of running applications?
Because running programs use RAM to run and not your disk. The only speed you see from the SSD is a quicker initial load of the software and that is just a fraction of time.
What can I do to make performance better?
Regarding the components...
Dell Inspiron 5570 i7
Get a better notebook. I9 will be quicker overall
with following specs 8GB RAM
Get memory that has quicker timings. It will not make a lot of difference though.
If your system is using swap add more memory. More memory can mean a slower boot but will solve swapping. If the system uses swap it will be noticeably slower than when it does not.
Otherwise there is no speed to gain here.
256GB SSD, 4GB Radeon Graphic Card, 2TB HDD.
Put the OS and
/home/ on the SSD for better boot and load boot times if not done already. These have nothing to do with speeding up running programs.
Still its performance is not good and the system hangs after 2-3 applications are open.
I do not see any proof in your question so what you are saying might be normal.
Why not include some statistics? Like results from
iotop Please add those to the question.