My computer boots fine, but after I log-in, the stuff on the taskbar load slow and surfing the web is very slow as well. What could be the problem?

Use software like HWINFO64 or Hardware Monitor to check temps while you are using the PC. This will also show you what kind of load and speeds you are seeing on the CPU.

Could you please list the make/model of laptop and the hardware specs of it?

Did it have a thermal pad that you removed when you applied the thermal paste? On a laptop, some of them will have an air gap if there's only paste replacing the pad, and need a metal shim added to make up the difference, or put a new thermal pad on, of the same thickness as the original. The gap is usually too large to only add more thermal paste.

The shim should be near the same thickness that the thermal pad was, or very slightly thicker. Ideal metal is copper, or at least aluminum. In a pinch I once lapped down an old (1982 or older are almost all copper) penny on both sides and used that. I put a plastic retainer piece (just sheet plastic with a hole cut in the middle) around it just in case the penny worked itself out. It wasn't my laptop, free repair for a friend so he got what he paid for. It was still working when he sold it months later.