I mean will my cpu be slow down by adding ssd to the laptop
Exactly.
A "bottleneck" does not 'slow down' a different component.
At worst, it does not let the other component reach its full potential.
For instance, a high end GPU may be said to be 'bottlenecked' by a low end CPU.
It does not slow the GPU down, just that it may not reach its full potential.
And that has nothing to do with CPU+ drive.
Yes, change that HDD for a 2.5" SATA III SSD of whatever size fits your budget. 250GB minimum.
Your system will be
faster, not slower.