Why is my laptop so slow?

Sadly, your systems RAM is quite limited with just 4GB. This is by far your biggest performance gain getting to a 8GB board MacBook Air 13" (Mid 2013-Early 2014) 1.4 GHz Logic Board sadly that’s the limit fo this series. The next biggest performance gain is getting a larger SSD OWC Aura Pro X2 SSD I would try to get to the 480GB as you need more free space with a limited RAM system as your system will use the SSD for VRAM.

Now the kicker! Are you willing to invest $450 into this system? You may want to look at getting a more powerful system (used) as an example a 2018 MacBook Air with 16GB of RAM with 256/512GB SSD.